AbstractSimultaneous measurements of stress and birefringence were made on a vulcanized styrene‐butadiene rubber (SBR) at 0°, 15° and 25°C. The stress‐strain curves at all temperatures showed a strong upward deviation from the Gaussian statistical theory at high extensions which became more pronounced with decreasing temperature due to the increasing extension to break. The birefringence, however, was substantially of the form predicted by the Gaussian theory. Comparison studies on natural rubber under similar conditions of experiment showed birefringence and hysteresis effects which could be interpreted on the basis of strain‐induced crystallisation (as revealed by X‐rays). Such effects were completely absent in the SBR samples which also showed no evidence of crystallisation on X‐ray examination in the stretched state. It is concluded that the upturn in stress‐strain curve in both materials is primarily due to the limited extensibility of the network, as treated in the non‐Gaussian statistical theory, and that crystallisation, when present, plays only a