AbstractThe entropy correlation theory predicts that any structural change whichdecreasesthe configurational entropyScof a complex molecular system will lead to an approximately equalincreasein the activation entropy ΔS*for structurally controlled rate processes such as diffusion. In symbols, Δ(ΔS*) =‐ΔScor ΔS*(A) = ΔS*(1)‐ΔSc, where, in the present case, the ordering parameter A is the elongation ratioL/L0. Although the theory is believed to be of wider scope, this paper deals with its confirmation for the case of gas diffusion in permanently stretched polymeric membranes. The systems investigated included representative poly(alkyl methacrylates) (viz. methyl, ethyl, isobutyl, and normal butyl methacrylate and a copolymer), and a selection of nonpolar and polar gases (Ar, O2, N2, CO). Diffusion and permeation measurements on the oriented films were made by the Daynes‐Barrer time lag method using a mass spectrometer for the detection instrument. The degree of orientation and its stability during an experiment were characterized by measurements of optical birefringence. For a comparison of the general theory with the specific data, it was found useful to prepare the samples in such a way that ΔSc≈a(A2+ (2/A)–3, corresponding to a quenched elastomer; then the data could be represented such that logDOis a linear function of ΔSc, whereDOis the preexponental factor in the diffusion equati