The polymer—solvent interaction parameter for an ethylene—propylene ter-polymer containing 53 mole-% ethylene was determined as a function of degree of crosslinking in n-heptane and benzene. A well characterized EPR gum vulcanizate was used to obtain the polymer-solvent interaction parameter in fourteen solvents, and the cohesive energy density of the terpolymer was evaluated. The composite tensile creep curves for gum and filled EPR vulcanizates were obtained and compared with SBR creep curves.