A carbon molecular sieve (CMS) is a carbonaceous material with a narrow pore size distribution, which can separate molecules based on their size, shape, and adsorption kinetic rate. In this study, a commercial CMS was used to measure the adsorption kinetics of carbon dioxide and carbon monoxide. The rate of adsorption was investigated by considering two main resistances, surface barrier and diffusion. The results showed that molecular parameters, such as difference in shape, size, and interactions of molecules, lead to different adsorption kinetics mechanisms. In the system investigated in this study, the adsorption kinetics of both CO2 and CO sorbates were controlled by combined diffusion and surface barrier mechanisms, in which the surface barrier was found to be the main resistance to gas molecule uptake. Even though this study confirmed surface resistance as a dominant transfer mechanism, the systematic use of the combined model in the analysis provided further insights in the mass transfer due to adsorption of CO2 and CO molecules in the CMS adsorbent. The rate constants were found to follow the Darken equation for both sorbates. The kinetic selectivity of CO2 over CO was calculated from the combined surface barrier/diffusion model parametric analysis. The results generally showed a greater selectivity to carbon dioxide over carbon monoxide, i.e. higher mass transfer rates.