The evolution of morphology of reactive thermoplastic/unsaturated polyester blends at the surface of glass fibers is investigated during curing. The study focuses on two different types of thermoplastics, incompatible and compatible respectively with the polyester resin. Poly(dimethylsiloxane) and poly(methyl methacrylate) are chosen as incompatible thermoplastics, and poly(vinyl acetate) as a compatible thermoplastic. In the presence of incompatible thermoplastics, the blends form an emulsion during the entire course of curing. In that case, a correlation exists between the surface tensions of the components of the blend measured before curing and the final morphology at the surface of the fibers. For a compatible thermoplastic, on the other hand, a reactioninduced phase separation occurs during curing. In that case, the morphology at the surface of the fiber after phase separation cannot be fully determined by the surface tensions of the components.