The crosslinking (cure) of resins containing an unsaturated polyester from maleic and phthalic anhydrides and glycol mixed with styrene was investigated by 13C-NMR analysis of the hydrolysis products, as a function of the styrene monomer concentration in the resin. The 13C-NMR spectra of the copolymers between the fumarate moieties of the polyester and the styrene monomers, obtained from the hydrolysis of the ester bonds of the cured resin, revealed a pattern of resonances due to styrene ring quaternary carbon atoms; this result indicates the presence of different type of sequences in the polyester-styrene-polyester enchainments. The frequency of these enchainments depends on the styrene content in the original resin. Polystyrene bridges, i.e. (styrene) n, were found to be present for every fumarate-styrene molar ratio and primarily for fumarate; styrene molar ratios ⩽1.0. An average ñ value was estimated as 4.5.