Some experimental results from the literature are difficult to explain by the existing diffusion theories using a concentration gradient or a swelling rate. A new diffusion mechanism is proposed. in which the driving force for diffusion is the sorption of penetrant molecules onto polymer chains. Resistance of the matrix to the Row of penetrant opposes this force. Both forces are a direct consequence of molecular interactions. The interaction between the solvent molecules can be expressed by viscosity eta and by surface tension T-s. The interaction of solvent with polymer matrix can be described by wettability (or contact angle alpha). The molecular structure of the polymer and its density fluctuations (average capillary radius r) are also important. Swelling of the matrix will change the molecular geometry and thus solubility. The rate of diffusion can be expressed as nu = (T-s/4 eta )rcos alpha 1/(x(0) + B) and the diffusion distance as x(0) = B((kr + 1)(1/2) - 1). where B and k are diffusion constants. The concentration dependence on distance and time is well described by an empirical equation C/C-0 = exp (-(x/x(0))(ax0)), where constant a describes the sharpness of the diffusion front. This model is applicable to swelling or non-swelling polymers, gels and porous media and can so far explain all the observed features of the diffusion process. (C) 2001 Elsevier Science Ltd. All rights reserved.