1. Structural analysis by the oxidation method was carried out on chloromethylation products from ethyl phenylacetate, benzyl chloride, ethylbenzene, cumene, tert-butylbenzene, and all four monohalobenzenes. In all cases, apart from that of tert-butylbenzene, it was found that mixtures of ortho and para isomers were formed. The relative amounts of the isomers were determined and were found to depend greatly on the character of the substituent. 2. In the chloromethylation of toluene, ethylbenzene, cumene, and tert-butylbenzene, the content of para isomer in the mixture progressively increases; this is to be explained by the effect of the size of the substituent (steric factor). In the chloromethylation of halobenzenes, the amount of para isomer progressively diminishes as we pass from fluorobenzene to iodobenzene, this being determined by the ionizing effects of the halogen atoms. 3. On the basis of our results and data in the literature on reaction rates, it is concluded that chloromethylation is an electrophilic-substitution reaction.