For the Contracting States : BE, CH, DE, FR, GB, IT, LI, LU, NL, SE 1. 1,2-Dithiol-3-one derivative, characterized in that it corresponds to the general formula : see diagramm : EP0119895,P26,F1 in which - either R denotes a hydrogen or chlorine atom, A denotes an oxygen atom or an imino, alkylimino, alkylcarbonylimino or formylimino radical and Ar denotes a phenyl radical substituted by a cycloalkyl radical in which the alkyl part contains 3 to 7 carbon atoms, an alkenyl radical containing 2 to 4 carbon atoms as a straight or branched chain, an anilino, formyl, semicarbazonomethyl, 1,3-dithiolan-2-yl, 1,3-dioxolan-2-yl, dialkylaminomethyleneamino, trifluoromethylthio or alkylcarbonyloxy radical or an alkyl radical substituted by a hydroxy, alkoxy, carboxy, amino, alkylcarbonylamino, alkoxycarbonylamino, dialkylaminomethyleneamino, formamido, alkylureido, phenylsulphonyl, carbamoyl, alkylcarbamoyl, dialklylcarbamoyl, formul, semicarbazonomethyl, 1,3-dithiolan-2-yl or 1,3-dioxolan-2-yl radical or Ar denotes a pyridyl, quinolyl, isoquinolyl, indanyl or quinoxalinyl radical, - or R denotes a hydrogen atom, A denotes an oxygen atom or an amino, alkylimino, alkylcarbonylimino or formylimino radical and Ar denotes a phenyl radical substituted by an alkyl radical, - or R denotes a chlorine atom, A denotes an oxygen atom or an alkylcarbonylimino or formylimino radical and Ar denotes a phenyol radical substituted by an alkyl radical, it being understood that, unless specially mentioned, all the alkyl radicals and alkyl moieties contain 1 to 4 carbon atoms as a straight or branched chain, as well as, where these exist, its salts of addition with acids, its metal salts and its salts of addition with organic bases. For the Contracting State : AT 1. Process for the preparation of 1,2-dithiol-3-one derivative, of general formula : see diagramm : EP0119895,P28,F1 in which - either R denotes a hydrogen or chlorine atom, A denotes an oxygen atom or an imino, alkylimino, alkylcarbonylimino or formylimino radical and Ar denotes a phenyl radical substituted by a cycloalkyl radical in which the alkyl part contains 3 to 7 carbon atoms, an alkenyl radical containing 2 to 4 carbon atoms as a straight or branched chain, an anilino, formyl, semicarbazonomethyl, 1,3-dithiolan-2-yl, 1,3-dioxolan-2-yl, dialkylaminomethyleneamino, trifluoromethylthio or alkylcarbonyloxy radical or an alkyl radical substituted by a hydroxy, alkoxy, carboxy, amino, alkylcarbonylamino, alkoxycarbonylamino, dialkylaminomethyleneamino, formamido, alkylureido, phenylsulphonyl, carbamoyl, alkylcarbamoyl, dialklylcarbamoyl, formyl, semicarbazonomethyl, 1,3-dithiolan-2-yl or 1,3-dioxolan-2-yl radical or Ar denotes a pyridyl, quinolyl, isoquinolyl, indanyl or quinoxalinyl radical, - or R denotes a hydrogen atom, A denotes an oxygen atom or an amino, alkylimino, alkylcarbonylimino or formylimino radical and Ar denotes a phenyl radical substituted by an alkyl radical, - or R denotes a chlorine atom, A denotes an oxygen atom or an alkylcarbonylimino or formylimino radical and Ar denotes a phenyol radical substituted by an alkyl radical, it being understood that, unless specially mentioned, all the alkyl radicals and alkyl moieties contain 1 to 4 carbon atoms as a straight or branched chain, as well as, where these exist, its salts of addition with acids, its metal salts and its salts of addition with organic bases, characterized in that a) for the preparation of a product of general formula (I) in which A denotes an oxygen atom or an imino or alkylimino radical, R denotes a chlorine atom and Ar is defined as above except for denoting a phenyl radical substituted by a dialkylamino-methyleneamino radical or an alkyl radical substituted by a dialkylaminomethyleneamino radical, a product of general formula : Ar-A-H in which Ar and A have the corresponding definition, is reacted with 4,5-dichloro-1,2-dithiol-3-one of formula : see diagramm : EP0119895,P28,F2 and the product obtained is then isolated and converted, if appropriate, into a salt of addition with an acid, into a metal salt or into a salt of addition with a nitrogenous base, or in that b) for the preparation of a product of general formula (I) in which A denotes an alkylimino radical and the other symbols are defined as above, a product of general formula : R1 X in which R1 denotes an alkyl radical and X denotes a halogen atom or a reactive ester residue such as mesyloxy or tosyloxy, is reacted with a product of general formula (I), in which A denotes an imino radical and the other symbols are defined as above, that is to say a product of general formula : see diagramm : EP0119895,P29,F3 and the product obtained is then isolated and is converted, if appropriate, into a salt of addition with an acid, into a metal salt or into a salt of addition with an organic base, or in that c) for the preparation of a product of general formula (I) in which A denotes an alkylcarbonylimino or formylimino radical and the other symbols are defined as above, a mixed anhydride of general formula : R2 COOCOCH3 in which R2 denotes a hydrogen atom or an alkyl radical, is reacted with a product of general formula (I) in which A denotes an imino radical and the other symbols are defined as above, that is to say a product of general formula : see diagramm : EP0119895,P29,F4 and the product obtained is then isolated and is converted, if appropriate, into a salt of addition with an acid, into a metal salt or into a salt of addition with a nitrogenous base, or in that d) for the preparation of a product of general formula (I) in which R denotes a hydrogen atom and the other symbols are defined as above, a product of general formula (I) in which R denotes a chlorine atom and the other symbols are defined as above, that is to say a product of general formula : see diagramm : EP0119895,P29,F5 is dechlorinated and the product obtained is then isolated and is converted, if appropriate, into a salt of addition with an acid, into a metal salt or into a salt of addition with a nitrogenous base, or in that e) for the preparation of a product of general formula (I), in which Ar denotes a phenyl radical substituted by a 1,3-dithiolan-2-yl or 1,3-dioxolan-2-yl radical or an alkyl radical substituted by a 1,3-dithiolan-2-yl or 1,3-dioxolan-2-yl radical and the other symbols are defined as above, a product of general formula : H-Y(CH2 )2 -Y-H in which the symbols Y both denotes a sulphur or oxygen atom, is reacted with a product of general formula (I), in which Ar denotes a phenyl radical substituted by a formyl radical or by an alkyl radical substituted by a formyl radical, that is to say a product of general formula : see diagramm : EP0119895,P29,F6 in which Q denotes a valency bond or an alkylene radical containing 1 to 4 carbon atoms as a straight or branched chain, and the other symbols are defined as above, and the product obtained is then isolated, or in that f) for the preparation of a product of general formula (I), in which Ar denotes a phenyl radical substituted by a dialkylaminomethyleneamino radical or by an alkyl radical substituted by a dialkylaminomethyleneamino radical and the other symbols are defined as above, a product of general formula : see diagramm : EP0119895,P29,F7 in which the radicals R3 , R4 , R5 and R6 denote alkyl radicals is reacted with a product of general formula (I), in which Ar denotes a phenyl radical substituted by an amino radical or by an alkyl radical substituted by amino radical, that is to say a product of general formula : see diagramm : EP0119895,P29,F8 in which Q denotes a valency bond or an alkylene radical containing 1 to 4 carbon atoms as a straight or branched chain and the other symbols are defined as above, and the product obtained is then isolated and is converted, if appropriate, into a salt of addition with an acid, or in that g) for the preparation of a product of general formula (I), in which Ar denotes a phenyl radical substituted by an alkyl radical which is itself substituted by an alkylcarbonylamino or alkoxycarbonylamino radical and the other symbols are defined as above, a product of general formula : R7 X1 in which R7 denotes an alkylcarbonyl or alkoxycarbonyl radical and X1 denotes a halogen atom, is reacted with a product of general formula (I), in which A denotes a phenyl radical substituted by an alkyl radical substituted by an amino radical and the other symbols are defined as above, that is to say a product of general formula : see diagramm : EP0119895,P30,F9 in which Q' denotes an alkylene radical containing 1 to 4 carbon atoms as a straight or branched chain and the other symbols are defined as above, and the product obtained is then isolated, or in that h) for the preparation of a product of general formula (I), in which Ar denotes a phenyl radical substituted by an alkyl radical substituted by a formamido radical and the other symbols are defined as above, an alkyl formate of general formula : HCOOR8 in which R8 denotes an alkyl radical, is reacted with a product of general formula (I), in which Ar denotes a phenyl radical substituted by an alkyl radical which is itself substituted by an amino radical, that is to say a product of general formula : see diagramm : EP0119895,P30,F10 in which Q' denotes an alkylene radical containing 1 to 4 carbon atoms as a straight or branched chain and the other symbols are defined as above, and the product obtained is then isolated, or in that i) for the preparation of a product of general formula (I), in which Ar denotes a phenyl radical which is itself substituted by an alkyl radical substituted by a ureido radical and the other symbols are defined as above, an isocyanate of general formula : OCNR9 in which R9 denotes an alkyl radical, is reacted with a product of general formula (I), in which Ar denotes a phenyl radical substituted by an alkyl radical which is itself subst