Blends of unsaturated polyester resin (UP) and polyethylene terephthalate oligomers (PETO) have been prepared with different relative compositions. These materials form a kind of semiinterpenetrating polymer networks (semi-IPN’s) in which the microstructure and the tensile properties are shown to depend on composition. Impact strength of the blends is also reported, and the performance of the blends was found to be superior to pure UP.