Abstract It has been shown, both theoretically and experimentally, that in simple elongation of a sample of pure-gum vulcanizate, an approximately linear relationship between the tensile stress t and the birefringence (n1−n2) obtains, provided the strain is not sufficiently large to produce crystallization. We may write: (n1−n2)=Ct, where C, the constant of proportionality, is termed the stress coefficient, and is theoretically independent of the degree of cross-linking (vulcanization) of the vulcanizate, and should be identical for all pure rubbers.