A study has been made of the influence of a dilating liquid, present during vulcanization, on the stress–strain behavior of EPT vulcanizates. Stress–strain measurements performed on the vulcanizates after extraction of the dilating liquid, taken over a range of strain rates, indicate the effective absence of time-dependent behavior and suggest that a close approach to equilibrium has been achieved. From the experimental data, values of the C1 and C2 constants of the empirical Mooney-Rivlin equation were derived. It was found that both C1 and C2 decreased with increased precure dilation, C2 decreasing more rapidly than C1. The observed decrease in C1 is less than would be predicted by simple analogy with the Gaussian modulus; it is suggested that this difference might be explained by changes in network configuration and topology associated with the precure dilation.