Methyl silanes of the types (CH₃)_nSiCl_4-n and (CH₃)_mSi(CH₂Cl)Cl_3-m have been photochemically chlorinated in the vapor phase.It was found that the structure of the dichlorinated silanes formed was greatly influenced by the presence of Si}CI bond.Silanes with no Si-Cl bond behaved similarly as the hydrocarbons,i.e., the substitution of two Cl-atoms for hydrogen occurred on different C-atoms in preference to the same C-atom, while the presence of the SiCl bond made the reverse to.be true, the gem-dichloro-compound predominated.According to the authors' opinion, this discrepency is due to the fact that the Si-Cl bond exerts an inhibitive effect on the a-methyl group in the event of chlorination, and, at the same time, a promotive effect on the a-chloromethyl group so as to make it easier to be further chlorinated.