Thermoset materials obtained from styrene/vinyl ester resins of different molecular weights modified with poly(methyl methacrylate) (PMMA) were prepared and studied. Scanning electron microscopy and transmission electron microscopy micrographs of the fracture surfaces allowed the determination of a two-phase morphology of the modified networks. Depending on the molecular weight of the vinyl ester oligomer, the initial content of the PMMA additive, and the selected curing temperature, different morphologies were obtained, including the dispersion of thermoplastic-rich particles in a thermoset-rich matrix, cocontinuous structures, and the dispersion of thermoset-rich particles in a thermoplastic rich matrix (phase-inverted structure). Density measurements were performed to determine the effect of the PMMA-modifier concentration and curing temperature on the volume shrinkage of the final materials. The development of cocontinuous or thermoplastic-rich matrices was not too effective in controlling the volume shrinkage of the studied vinyl ester systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness showed that the addition of PMMA increased the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl ester networks.