Thermoset blends based on unsaturated polyester and triblock copolymers containing poty(ethylene oxide) and poly(propylene oxide) blocks were investigated. No evidence of self-organization was found. Under our experimental conditions, the block copolymers behaved like classical thermoplastic additives used for shrinkage compensation. According to the type and content of the copolymer, initial systems with one phase or two phases were generated. During curing, except in one case in which thermally induced phase separation was found, reaction-induced phase separation was observed, leading to various types of cured morphologies (cocontinuous or dispersed). For a cocontinuous, two-phase morphology, experimental observations revealed phase separation proceeding via spinodal decomposition frozen in the early stage by gelation. These triblock copolymers appear to be interesting new additives that could be used as good shrinkage compensator additives for industrial applications because it is known that a cocontinuous morphology is needed for this purpose. As for the final morphologies containing dispersed phases, the interpretation of the phenomena occurring upon polymerization requires further investigation. No clear discrimination between spinodal decomposition and nucleation and growth was achieved; in some cases, various successive phase separations were observed. The curing kinetics were examined: an increase in the molar weight and/or copolymer content led to a lower reaction rate. Kinetics curves were modeled by the semiempirical autocatalytic reaction model of Kamal and Sourour with a diffusion-control function. The gelation and vitrification of the system played major roles in the reaction kinetics. (c) 2006 Wiley Periodicals, Inc.