In a process for the preparation of a metal phthalocyanine which comprises heating in an inert organic diluent which is liquid at the reaction temperature, a reaction mixture containing an aromatic phthalocyamine intermediate of the group consisting of aromatic o-dicarboxylic acids, the corresponding esters, anhydrides, salts (especially ammonium salts), mono- and di-amides, esters and ammonium salts of the monoamides; imides, iminoimidines, corresponding o-cyanocarboxylic acids and their amides, esters, and ammonium salts, corresponding o -polychloro- or o -polybromo-o-dimethyl aromatic compounds and o -chloro- or o -bromo-methyl-o-cyano aromatic compounds, and their nuclear substitution products other than those containing additional acid groups such as sulpho or carboxyl groups, with a phthalocyanine forming metal-yielding reagent, and a nitrogen donor of the group consisting of urea, biuret, guanidine, guanidyl urea, dicyandiamide and cyanuric acid, there is included in the reaction mixture an aromatic monobasic sulphonic acid, a salt thereof (especially a salt with an alkali metal, alkaline earth metal, ammonium, or phthalocyanine forming metal, ester thereof, or the corresponding acid halide, in an amount corresponding to at least 25 per cent of the weight of said aromatic phthalocyanine intermediate, the monobasic aromatic sulphonic acid or derivative used being itself incapable of forming a phthalocyanine compound under the reaction conditions and also incapable of reacting with the phthalocyanine compound formed (except for yielding a salt-forming metal thereto) nor with the ortho-substituted aromatic intermediate therefor in such manner as to combine therewith or to modify the organic structure of the resulting phthalocyanine compound. A catalyst containing an element of Group V or VI of the periodic system of atomic number from 15 to 92, e.g. an alkali metal or ammonium molybdate, phosphomolybdate, or tungstomolybdate, is preferably included in the reaction mass. The reaction is generally carried out at 150-210 DEG C. Specified aromatic sulphonic acids and derivatives include the sodium, potassium, calcium, strontium, barium, copper, nickel, iron, cobalt, tin, magnesium, aluminium, zinc, and vanadium salts of benzene sulphonic acid, of m-nitrobenzene sulphonic acid, of naphthalene-a or-b -sulphonic acid, of 8-chloro-1-naphthalene sulphonic acid, the corresponding free acids, acid chlorides such as benzene sulphonyl chloride, sodium diisopropyl-or diisobutyl naphthalene sulphonate, the corresponding free acid, and benzene sulphonic acid methylester. The aromatic phthalocyanine-forming intermediate may be substituted by halo, nitro, alkyl, aryl, condensed nuclear aryl, aryloxy, alkoxy, arylthio, alkylthio, or alkyl or aryl keto groups (acyl groups). Specified organic diluents are those which remain liquid under the conditions of reaction, e.g. trichlorobenzene, dichlorobenzene, nitrobenzene, naphthalene, chlorinated naphthalenes, quinoline, and benzophenone. The phthalocyanine-forming metal yielding reagents specified are polyvalent metals and their salts such as those of copper, aluminium, magnesium, nickel, iron, cobalt, zinc, tin, and vanadium, suitable compounds including the chlorides, bromides, sulphates, nitrates and oxides. The amount of monobasic aromatic sulphonic acid or derivative used is preferably about one-half to two-thirds of the weight of the phthalocyanine-forming intermediate. In examples (1) chloroaluminium phthalocyanine is obtained by forming a slurry of phthalic anhydride, urea, ammonium molybdate and sodium xylene sulphonate in trichlorobenzene, adding a slurry of anhydrous aluminium chloride in trichlorobenzene, heating the mixture under reflux for 6 hours, and then adding a slurry of urea in trichlorobenzene and maintaining the temperature at 200-205 DEG C. for 5 hours. Calcium, magnesium or aluminium benzene sulphonate may be used instead of sodium xylene sulphonate; (2) hexadecachlorocopper phthalocyanine is obtained by heating a slurry of tetrachlorophthalic anhydride, sodium xylene sulphonate, cuprous chloride, urea and ammonium molybdate at 200 DEG to 205 DEG C. for 5 hours; (3) a slurry of phthalamide, urea, aluminium trichloride, sodium xylene sulphonate and ammonium molybdate in tetrachlorobenzene is heated at 200-205 DEG C. for one hour and a slurry of urea in trichlorobenzene then added and the mixture agitated at 200-205 DEG C. for 5 hours to yield chloroaluminium phthalocyanine. Phthalimide or o-cyanobenzamide may be used instead of phthalamide; (4) a slurry of phthalic anhydride, urea, anhydrous nickel chloride, sodium xylene sulphonate and ammonium molybdate in trichlorobenzene is heated at 200 DEG C. for one hour, a slurry of urea in trichlorobenzene is then added and the mixture heated at 200 DEG C. for 4 hours to yield nickel phthalocyanine; (5) magnesium phthalocyanine is obtained by heating a slurry of phthalic anhydride, urea, magnesium oxide, sodium xylene sulphonate (or magnesium benzene sulphonate) and ammonium molybdate at 200 DEG C. for 4 hours. It is stated that metallic copper or other cuprous salts can be used instead of cuprous chloride, aluminium sulphate or acetate instead of aluminium chloride, magnesium nitrate or chloride instead of magnesium oxide and nickel nitrate instead of nickel chloride and that salts of cobalt, metallic zinc or its salts, iron or iron salts can be substituted for the metal compounds of the examples to yield the corresponding metal phthalocyanines. U.S.A. Specification 2,214,477 and U.S.A. Application 403,866 published by the Alien Property Custodian of the U.S.A. are referred to.