The kinetics of methanol vapour sorption and concurrent longitudinal swelling of cellulose acetate membranes has been studied and interpreted in terms of ’viscous’ swelling and differential swelling stress effects, on the basis of previous modelling work. The close parallelism of the kinetic behaviour observed here and in a previous study of the cellulose acetate-acetone system, shows that both sorption and longitudinal swelling kinetics can be understood in terms of the viscoelastic response of the plasticized polymer to an applied osmotic stress, without regard to the nature of the penetrant causing the osmotic stress and plasticization. It has also been shown that, from the first stage of two-stage absorption curves, diffusion coefficient values can be derived agreeing with those evaluated by Takizawa et al. from steady state permeation and equilibrium sorption data.