Vinyl esters are used widely as thermoset matrix materials for reinforced composites; however, they suffer from low-impact resistance. Substantial enhancement of the toughness of brittle polymers may be achieved by dispersing elastomeric inclusions or rubber particles in the polymer matrix, inducing multiple crazing and shear yielding of the matrix. The main objectives of this work are morphological characterization of vinyl ester/reactive rubber systems and investigation of the mechanical and fracture behavior of these systems. Additional studies focused on rubber endcapped vinyl ester in the absence and presence of added reactive rubber. The initial compatibility of the liquid rubber with the liquid resin was studied. This is a key factor, along with cure conditions, in determination of the possible morphologies, namely, the degree of phase separation and particle size. The initial rubber/resin compatibility was found poor and all attempts to improve it by means of surfactants or ultrasonic treatment have not been successful. The flexure mechanical and fracture behavior of the cured resin/rubber systems was investigated. Three basic types of crack propagation behavior, stable, unstable, and stick-slip, were observed. Fracture toughness of various resin/rubber systems was evaluated and was found to increase with increased content of rubbery second-phase material. However, there is some payoff in stiffness and flexural strength of the cured resins. The addition of rubber does not affect the resin toughness at impact conditions. Analysis and interpretation of fractures morphology show that both multiple crazing and external cavitation play an important role in the fracture mechanism of the rubber modified specimens. No shear yielding is evident.