The final morphology of cured blends based on unsaturated polyester, styrene, and low-molar-weight saturated polyester as a low profile additive (LPA) was investigated with atomic force microscopy and scanning electron microscopy. The observed structure was compared to those obtained with widely used poly(vinyl acetate) (PVAc). On the surface and in the bulk, a network of particles, ranging in size from 50 to 60 nm, was observed with saturated polyester as an LPA. The influence of the molar weight and LPA content was investigated. To determine the mechanism of formation of such a morphology, in situ experiments were carried out to elucidate the phase-separation mechanism. Small-angle laser light scattering and small-angle neutron scattering experiments were performed on ternary blends containing PVAc and saturated polyester, respectively. The first stage of spinodal decomposition was observed in both cases. Within our experimental conditions, gelation froze further evolution and led to a two-phase cocontinuous structure that imposed the final morphology; characteristics. In particular, the period and amplitude of the concentration fluctuations generated during the phase separation played essential roles. (C) 2005 Wiley Periodicals, Inc.