Real-time dielectric relaxation and theological measurements are reported during the course of cure for three thermoplastic-modified thermoset resin blends. Two thermoset resin systems were used, one containing only a difunctional epoxy and the other containing a mixture of tri- and difunctional epoxies. The latter mixture was used to react two reactively terminated polysulfone thermoplastics with slightly different molecular weights. Positron annihilation measurements are reported on one of the thermoplastic-thermoset resin systems as a function of the thermoplastic composition, and indicate that changes in composition lead to variations in the free-volume content. Mechanical properties are presented for all three systems. These data indicate that the phase structure has a significant effect on mechanical performance, but the most dominant effect results from change in the thermoset matrix material.