An epoxy-amine thermosetting polymer, modified by 15 wt% castor oil (CO) or by a commercial rubber (epoxy-terminated butadiene-acrylonitrile copolymer, ETBN), was cured in a mold at different heating rates. CO-modified materials cured at high heating rates exhibited a nonuniform phase separation profile, i.e. a single-phase transparent core surrounded by opaque layers containing dispersed domains, The resulting structure was explained by simulating conversion vs. temperature trajectories taking place during the cure. Thermal and mechanical properties of uniform and nonuniform materials were compared. These include glass transition temperature, microhardness, yield stress and yield strain in uniaxial compression, and fracture toughness. Some possibilities for further research in this area are discussed.