The study of the stress-temperature curves of ethylene-propylene copolymers having different compositions revealed that ethylene-rich samples crystallize under stretching at room temperature. The results obtained suggest that crystallization is probably the outcome of two phenomena: orientation of the microcrystallites, undetectable by x-ray analysis, and true crystallization under stretching. These phenomena are accompanied by strong negative variations of internal energy; these variations, when occurring at low stretching ratios, cannot be distinguished from the variations of conformational internal energy. Therefore, when calculating the ratio ƒE/ƒ in samples containing equimolecular amounts of the two monomers, and even more in the ethylene-rich ones, at room temperature, the values may be affected by uncertainty due to the occurrence of crystallization. The calculation of ƒE/ƒ at high temperature, where the rubbery network follows Equation (2) perfectly, even in ethylene-rich samples, shows that this ratio is always negative and depends on the stretching ratio α, for any composition.