A combination of dynamic mechanical and swelling properties has been used to elucidate the structure of epoxy resin systems as a function of time and temperature of cure, concentration of flexibilizer, and curing agent concentration. The dynamic properties of the resin in the rubbery region were used to determine the molecular weights between cross-links. The poor network structure formed with epoxies is shown by the dynamic and swelling tests. The existence of heterogeneity or two phases has been found in the epoxy resin system under certain polymerization conditions. Broad damping peaks may not be due to chemical heterogeneity but may be the result of the curing reaction continuing during the tests.