A series of translucent interpenetrating polymer networks (IPNs) made of a reactive elastomer [linear (D) and branched (T) with varying molecular weights] (Jeffamine(TM)), a commercially available epoxy (D.E.R. 331), and an unsaturated polyester (15:85 wt %) were prepared. DSC data indicated complete cure after 8 h at 90degreesC. DMTA data showed a single glass-transition temperature (T-g) for all elastomer-containing IPNs, an indication of homogeneity. As expected, all IPNs showed a decrease in Tg with incorporation of elastomer, from 16 to 114degreesC or lower, the largest decrease being with T-5000. Izod impact strengths were increased by 28-44%, but with no apparent pattern among structure and molecular weight variations. In several cases the standard deviation of impact data increased significantly. Flexural data were measured using a three-point bend test. The highest flexural modulus obtained was that which incorporated linear D-2000 with a decrease of only 22% upon incorporation of the elastomer, whereas other compositions dropped up to 55% in flexural modulus. The strongest material obtained was that using D-2000 with a flexural strength increase of 65% upon incorporation of the elastomer. Two of the three branched elastomer components showed flexural strength increases of about 53%, but one was only equal to the base polyester resin. TGA data were recorded for all IPNs and values compared well to that of the pure polyester resin, with the exception of T-403, which showed a 20degreesC decrease, and D-2000 with a 10degreesC decrease.