Five networks based on a diglycidylether of bisphenol A-dimethacrylate prepolymer of molar mass 1100 g· mol−1crosslinked with 35%, 40%, 45%, 50%, and 55% styrene mass fractions were characterized by Fourier transform infrared spectroscopy, cross-polarization/magic-angle spinning, nuclear magnetic resonance spectroscopy, styrene extraction, and rubber elasticity measurements. Various structural models were built up from the available analytical data, and their crosslink density was compared to the experimental data derived from rubber elastic moduli. Globally, network models based on a purely statistical approach but taking into account the incomplete cure conversion make it possible to predict the trends of modulus variations. The kinetic data on copolymerization show, however, that the material is diphasic, one phase being composed of the vinylester network and the other of practically pure polystyrene. We tried to take this heterogeneity into account in modulus predictions using a simple mixture rule, but the method failed for high styrene mass fractions (s≫ 0.45), for which the experimental modulus value is about 50% higher than the predicted one. The results suggest the existence of a morphological transition for a critical styrene weight fraction between 40% and 45%.