An unsaturated polyester (UP) resin modified with a liquid polymer, polyoxypropylenetriamine (POPTA), at a concentration of 10 wt% has been precured at several temperatures. Phase separation takes place before gelation at all precure temperatures used. The glass-transition region has been analyzed by dynamic mechanical analysis. Mechanical properties have been related to microstructural features. With a precure temperature fixed, the unsaturated polyester CUP) resin has also been modified with different contents of POPTA. Fracture toughness of the mixtures has also been analyzed and results are compared to those for the unmodified mixture.