Analysis of the permeation of adsorbed molecules through a carbon pellet has been undertaken in order to investigate the dependence of the adsorbed phase mobility upon the concentration of the adsorbed phase. The dependence is shown to be described well by the model proposed by Chen and Yang [Y. Chen, R.T. Yang, Carbon 36 (10) (1998) 1525] which can apply to adsorbed phase concentrations greater than that at saturation but still within the BET limits. The time lag for permeation is also shown to provide a useful and reliable means for investigating the adsorbed phase diffusion process.