Aromatic chlorosilanes are prepared by reacting naphthalene or a benzenoid hydrocarbon free from aliphatic unsaturation in any side chains with an organo-dichloromonohydrosilane of the general formula RHSiCl2, wherein R is a monovalent hydrocarbon radical free from aliphatic unsaturation at a reaction temperature above 150 DEG C., at least a portion of the reaction mixture being in condensed phase, in the presence of a silicon-borate complex selected from the group consisting of (1) the reaction product of a silane of the general formula R14-a-bHaSiXb, where each R1 is a monovalent hydrocarbon radical, each X is alkoxy or chlorine, a is 0 or 1, b is an interger from 1 to 4, the sum of a and b being not greater than 4 with B2O3 or a compound of the general formula (R11O)xBR3-x, where R11 is a monovalent hydrocarbon radical or hydrogen, R is a monovalent hydrocarbon radical and x is 1, 2 or 3, but excluding a reaction product of organo - dichloromono - hydrosilane with B2O3 or H3BO3 formed in situ, and (2) the reaction product of a siloxane or a silanol with a compound of the general formula RyBY3-y, where R has the above significance, Y is a halogen atom and y is 0, 1 or 2, the complex being employed in such a proportion that at least 0.01 per cent by weight of boron is present calculated on the total weight of the reaction mixture. The preferred reaction temperature is from 150 DEG to 250 DEG C. A monosilyl derivative of the hydrocarbon is obtained using a slight excess of the hydrocarbon; the use of more than one molecular equivalent of the silane per mol. of the hydrocarbon increases the yield of polysilyl derivatives of the hydrocarbon, e.g. bis- and tris-silyl derivatives, without appreciable decrease in the amount of monosilyl derivative formed. Benzenoid hydrocarbons which may be used are benzene, polyphenyls such as p diphenyl and alkyl substituted benzenes such as toluene, xylene, cumene and mesitylene. Organo-dichloromonohydrosilanes that may be used are methyldichlorosilane and phenyldichlorosilane. The silicon-borate complex may be formed in situ or may be prepared in advance and added to the reaction mixture. When the complex is formed in situ, it may be formed from the organo-dichloromonohydrosilane used in the reaction, and the boron compound used is preferably of the general formula (R11cO)3B2-c where R11 is a monovalent hydrocarbon radical or hydrogen and c is 0 or 1. H3BO3 and B2O3 may be used for the formation in situ of the silicon-borate complexes. The silicon-borate complexes may be of a simple type, e.g. [(CH3)3SiO]3B, or may be of a more complex type, e.g. compounds containing any of the structures <FORM:0694526/IV (b)/1> , <FORM:0694526/IV (b)/2> , or <FORM:0694526/IV (b)/3> in which the open bonds may be satisfied by any of the group consisting of phenyl, methyl, alkoxy, chlorine and hydrogen, or may be linked to other silicon or boron atoms through oxygen. Examples of suitable silanes which may be used to form the silicon-borate complexes are SiCl4, HSiCl3, CH3HSiCl2, (CH3)2Si(OC2H5)Cl, (CH3)3SiOC2H5, (CH3)3SiCl, C6H5SiCl3, C6H5CH3SiCl2 and C6H5(CH3)2SiCl. If an alkoxy substituted silane is used, it is preferable that the alkoxy group contains less than 5 carbon atoms. The boron employed for the preparation of the complexes may be in the form of boric acid, boric anhydride or organic borates such as tributylborate, phenylboric acid: C6H5B(OH)2, and diphenylboric acid: (C6H5)2BOH. The silicon-borate complexes may also be formed by the interaction of siloxanes or silanols with boron halides or organo-boron halides of the above general formula such as phenylboron dichloride and diphenylboron chloride. When silicon halides are also present, the complexes may be formed from metallic boron or the boron hydrides. When the complex is previously prepared, it may be formed at or below reflux temperature at atmospheric pressure, and an excess of the silane may be used as a solvent for the complex. The use of B2O3 or the use of low-boiling silanes such as SiCl4 and HSiCl3 entails the use of temperatures higher than reflux temperatures with the aid of super-atmospheric pressures. In the examples: (1) benzene, methyldichlorosilane and tris-(trimethylsilyl) borate (prepared by heating boric acid and trimethylchlorosilane) are heated in a sealed bomb to give methyl phenyldichlorosilane; (2) methyl tolyldichlorosilanes are prepared by heating in a sealed bomb toluene, methyldichlorosilane and a boron-silicate catalyst obtained by heating boric acid and phenyldimethylchlorosilane; (3) methyl phenyldichlorosilane is obtained by heating in a sealed bomb benzene, methyldichlorosilane and a boron - silicate catalyst obtained by heating boric acid and methyldichlorosilane; (4) methyl phenyl dichlorosilane is obtained by heating tributylborate, benzene and methyldichlorosilane in a sealed bomb; (5) methyl phenyldichlorosilane is prepared by heating in a sealed bomb benzene, methyldichlorosilane and a boron-silicate catalyst consisting essentially of the compound (Me2EtOSiO)3B and obtained by mixing dimethyldichlorosilane and dimethyldiethoxysilane in the presence of a few crystals of boric acid to form dimethylethoxychlorosilane which is then refluxed with boric acid; (6) phenyl trichlorosilane is prepared by heating benzene containing a small amount of siloxanes, trichlorosilane and boron trichloride; the distillation residue remaining after removing the reactants and desired product contains boron and is distilled to give a boron containing fraction which is heated in a sealed bomb with benzene and phenyldichlorosilane to yield diphenyldichlorosilane.