A series of semi-interpenetrating polymer networks (semi - IPN’s) based on different compositions of an acyclic polyethylene terephthalate oligomer and unsaturated polyester resin (UP) were prepared. The oligomer was dissolved in the resin containing styrene crosslinker. Later this mixture was crosslinked at room temperature using methyl ethyl ketone peroxide catalyst and cobalt naphtanate as promoter. The tensile strength of the IPN’s decreases as the concentration of oligomer increases, whereas, elongation to break increases. A characterization of the oligomer used is also presented.