Polysiloxanes bearing epoxy groups as lateral substituents were prepared by the hydrosilylation of 1-allyloxy-2,3-epoxypropane (allylglycidylether) with poly(hydrogenmethylsiloxane-co -dimethylsiloxane)s (D-H-D copolymers) of various compositions. To determine the optimal conditions of the hydrosilylation catalyzed by hexachloroplatinic acid, the kinetics of the reaction were investigated for the poly(hydrogenmethylsiloxane) homopolymer. The reaction was first order in platinum, first order in double bonds, and 0.5 in SiH. This kinetic law was consistent with the hypothesis that hydrogenmethylsiloxane dyads are much more reactive than isolated units, which may be explained by the simultaneous insertion of two vicinal SiH's in a binuclear complex of platinum. This peculiar type of neighboring effect was investigated further by comparison of the kinetics of the hydrosilylation of D-D-H copolymers of various compositions and D-H block lengths and was confirmed by the microstructure of a D-D-H copolymer (50/50) before and after partial hydrosilylation. Triad analysis by Si-29 NMR showed that the resulting copolymer was not statistical but contained a high proportion of isolated SiH. This explains why hydrosilylation proceeded in two steps: a fast reaction of D-H-D-H dyads and a slow reaction of the remaining isolated D-H units.