The permeation of argon gas through membranes of poly(cis/trans 2-phenyl-5-ethyl-5-acryloxymethyl-1,3-dioxacyclohexane) (PAEDP) as been measured in the vicinity of the glass transition temperature of this polymer (similar to T(g)approximate to 50 degrees C). Both the permeation and the diffusion coefficients show only a slight dependence on temperature while the membrane remains in glassy state, but exhibit a sharp increase with temperature in the rubbery state. Theoretical calculations of the diffusion coefficient were performed according to the transition-state approach, i.e., assuming that the diffusant path is independent of the structural relaxation in the polymeric matrix, as a function of the smearing factor Delta and temperature. Reasonably good agreement among theoretical and experimental values of the diffusion coefficient was obtained. Theoretical calculations were also performed for poly(cis/trans 2-phenyl-5-ethyl-5-methacryloxymethyl-1,3-dioxacyclohexane) (PMAEDP), the methacrylate analog of PAEDP, which indicate that the diffusion coefficient of glassy PMAEDP is lower than that of glassy PAEDP when the same temperature is taken as the basis of comparison, due to the higher values of T-g in methacrylate than in acrylate polymers which, in turn is a consequence of the rigidity conferred to the polymeric chain by the methyl group.