Carbon molecular sieve membrane (CMSM) is prepared from Kapton polyimide. The CMSM presents strong molecular sieving effect for gas molecules with different dimensions. CO2 sorption and permeation experiments were performed on the CMSM over a wide pressure range (up to 40 atm). The permeation time lag and steady state permeation flux were employed to characterize the concentration dependence of the diffusion coefficient of the adsorbate. A new functional form is proposed for the concentration dependence of the surface diffusivity [D-mu(C)/D-mu(0) = 1/(1 - theta)(n), where theta is the ratio of surface coverage and n greater than or equal to 1] to cope with the experimental observations. With the sorption -diffusion mechanism, both transient time lag and steady state flux indicates that the surface diffusivity of CO2 takes a concentration dependence much stronger than that described by the traditional HIO model, [i.e. D-mu(C)/D-mu(0) = 1/(1-theta)]. The reason for this abnormality is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.