In the photochlorination of arylaliphatic hydrocarbons (C6H5R; R = CzH5 ; n. C3H7 ; i. C3H7 ; n. C4Hg ; sec. C4H9 ; n. CsH I substitution only takes place in the side chain, and preferentially in the a position, as was showed in the previous paper. Now, in order to make clear which effect is operative-either the polar effect of the substituent or a complexation effect of the aromatic nucleus of the reagent, photochlorination of the title compounds was examinated in various solvents (CCI4, aromatics and CSz). Firstly, diluting by CCI4, it was observed that relative reactivity, r.r. (alb), of an hydrogen a versus another b, varies with the concentration and can be expressed as a linear function :including complexation constant K, , specific rate k,,, and relative reactivity koO/kbo at infinite dilution (whose numerical values can be calculated by means of the former formula), if R is CzH, ; n.C3H7; i.C3H7 ; sec.C4H9, and with a more sophisticated one:for the other terms, R = n. C4H9, n. C5H ,.With benzene, selectivity is enhanced but with chlorobenzene it is depressed. Finally, the best results concerning the selectivity are obtained by photochlorination in the presence of C S z . We mention here the most interesting result of cumene (R = i. C3H7), giving up to 98% of a-chloroisomer in the presence instead of 86% of the same in the absence of CS2.En examinant les resultats relatifs A la photochloration directe des hydrocarbures arylaliphatiques, que nous avons present& dans le m h o i r e precedent, nous avons constate, entre autres, la proportion preponderante d'isomeres a-chlores.A titre d'exemple, epinglons le cas du cumene oh les repartitions experimentales (R. E. 85,5 % a-C1 et 14,5 P-Cl) sont quasi inverskes vis-a-vis des rbpartitions statistiques (R. S. 14,3 Yo a-CI et 85,7 % P-Cl) et m&me tres Bloignkes de celles calculQs au moyen des coefficients de HASS (R.T. 44 % cr-CI et 56 % P-Cl). A notre avis, la difference considkable existant entre ces repartitions peut Qtre attribuQ A deux actions distinctes au moins : d'une part, a I'activation par le groupe phenyle mettant en jeu son effet polaire, et d'autre part a I'augmentation de selectivite resultant de la complexation du chlore atomique par le substrat lui-mhe, agissant par effet de milieu.Rappelons succinctement, a ce propos, I'action des solvants.