The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6-hexanediol dimethacrylate (HDDMA) and 2-ethylhexyl methacrylate (EHMA)] in a polystyrene (PS) matrix were studied. The aggregation state, vitreous or rubbery, of the monomer/matrix system and the intermolecular strength of attraction in the monomer/matrix and growing macro-radical/matrix systems are the principal factors influencing the kinetics and mechanism. For the PS/HDDMA system, where a relatively high intermolecular force of attraction between monomer and matrix and between growing macroradical and matrix occurs, a reaction-diffusion mechanism takes place at low monomer concentrations (< 30 -40%) from the beginning of the polymerization. For the PS/EKMA system, which presents low intermolecular attraction between monomer and matrix and between growing macroradical and matrix, the reaction-diffusion termination is not clear, and a combination of reaction-diffusion and diffusion-controlled mechanisms explains better the polymerization for monomer concentrations below 30-40%. For both systems, for which a change from a vitreous state to a rubbery state occurs when the monomer concentration changes from 10 to 20%, the intrinsic reactivity and k(p)/k(t)(1/2) ratio (where k(p) is the propagation kinetic constant and k(t) is the termination kinetic constant) increase as a result of a greater mobility of the monomer in the matrix (a greater k(p), value). The PS matrix participates in the polymerization process through the formation of benzylic radical, which is bonded to some extent by radical-radical coupling with the growing methacrylic radica, producing grafting on the PS matrix.