A comprehensive study of toughening unsaturated polyester and vinyl ester resins by addition of liquid rubbers was carried out by considering the effects of cure temperature and gel time on final resin/rubber morphology. The objective was to produce a dispersed rubber phase consisting of particles less than 15 mu m in diameter with the addition of limited amounts of rubber, so as not to seriously reduce the modulus and strength of the base resin. A variety of liquid rubbers was used including those based on poly(butadiene acrylonitrile), poly(epichlorohydrin), and two poly(acrylates). Fracture toughness of unmodified and rubber modified materials was measured using the compact tension (CT) test geometry. Significant improvements in fracture toughness were achieved with little to no change in Young’s modulus or glass transition temperature. With modest rubber additions, the fracture toughness increased up to 62% for the polyester resin and up to 116% for the vinyl ester resin. In general, fracture toughness increases with increases in volume fraction of rubbery second-phase particles. However, results suggest that two-phase particles may be more effective tougheners than single-phase particles. The toughening mechanism appears to depend on the type of rubbery particle morphology present.