The unsaturated polyesters derived from maleic anhydride and 2,2-di(4-hydroxypropoxyphenyl)propane were copolymerized with styrene in order to obtain the crosslinked materials. They were studied by means of high resolution C-13 n.m.r. in the solid state (CP/MAS). The structural changes upon crosslinking were followed by analysing proton spin-lattice relaxation time, T-1(H) and spin-lattice relaxation time in the rotating frame, T-1 rho(H) vs crosslinking temperature and styrene content. With the assumption of spin diffusion among protons of the constituent components of the network, the T-1 rho(H) and T-1(H) values were calculated using a model for linear additivity of relaxation rates. The theoretical values were considered as a measure of copolymer effect while the difference between calculated and experimental data were interpreted as being due to crosslinking effect. The results were discussed in terms of the adequacy of crosslinking.