Measurement of batch adsorption of carbon dioxide with a carbon molecular sieve (commercially manufactured Takeda 5A) indicates no molecular sieving action but instead, micropore diffusion is shown to be rate limiting the adsorption dynamics. Permeation measurement through the same pellets is also performed and steady state analysis indicates there is negligible adsorbed phase transport along the pellet and that the gas phase diffusion process is a combined Knudsen and viscous mechanism. Batch adsorption and permeation methods are critically compared for their utility in determining which mass transfer processes are relevant and the conditions under which each technique is most useful are given. (C) 2000 Elsevier Science Ltd. All rights reserved.