Aspects of the copolymerization of styrene and acrylonitrile have been investigated in solution in the presence of Aerosil and of microfibrous Sivol, both unmodified and with the surface modified by the chemical grafting of (methacryloxymethyl) methyldiethoxysilane. An increase has been found in the rate of hetergeneous copolymerizationm which occurs simultaneously with an increase in the molecular mass both of the copolymers grafted to the surface and also the ungrafted polymers; this is related to the gel-effect created by the filler in the reaction system. The modified Sivol surface has been found to have a selective effect on the composition of the styrene-acrylonitrile grafted polymer that is formed. It has been established that the relative reactivity of styrene in the graft copolymerization reaction is reduced as compared with that in the heterogeneous non-graft process and in the homogeneous process.