The structure of a carbon molecular sieve (CMS) membrane is characterized by the through pores and blind pores with non-linear sorption isotherms inside. Time-lag analysis was conducted for gas permeation in such a structure and mathematical formulations were derived for two cases. It is found that the pressure dependence of the time lag is dominated by the ratio of sorption affinities in the two types of pores. The experimental permeation data of pure component CO2 and N-2 measured on a CMS membrane were used to validate the model. It is found that, for the adsorbing species (CO2), the model is able to well describe the diffusion anomalies over a wide range of permeation pressure, while for the weakly adsorbing species (N-2), the model is inadequate to cope with the anomalies at the low end of permeation pressure. (c) 2007 Elsevier Ltd. All rights reserved.