We report the phase behavior, the molecular assembly, and the crystallization in blends of unsaturated polyester resin (UP, thermoset polymer) and low molecular mass polyethylene-block-poly(ethylene oxide) (EEO, 1400 g mol(-1)). Liquid blends containing 2.5-25% of EEO block copolymer were prepared. At a macroscopic level, all the prepared blends are homogeneous at room temperature, exhibiting LCST-type behavior upon heating, with macrophase separation occurring at 75 degrees C in the studied range of compositions. Curing at a temperature lower than 75 degrees C prevents macrophase separation and allows the fixing of the structure obtained in the initial liquid state. In these conditions, EEO platelets exhibiting a high aspect ratio (6 nm thick, 500 nm to 1 mu m long) constituted of a crystalline PE core, bordered by PEO domains, are observed in the cured state by TEM. For low EEO content, isolated lamellae are obtained. For higher content, aggregation of the lamellae is observed without real isotrope/nematic transformation. These observations are in close correspondence with the real-time SAXS-WAXS experiments that permit us to follow the molecular assembly of the block copolymer upon heating. DSC study of the blends indicates that PEO crystallization is suppressed, which confirms the miscibility of the PEO segments with the UP. Concerning PE blocks, a strong depression of the crystallization temperature is observed in the cured samples, suggesting a confinement effect.