A new model has been developed to consider the concentration dependence of Fickian diffusivities for mixtures adsorbed in nanoporous adsorbents. It is derived from irreversible thermodynamics and has clear meaning for the concentration dependence as in the Maxwell-Stefan theory. With the diffusivities obtained from binary systems, this model can be easily extended to multicomponent systems without further correlation of mixture data. A binary mixture of carbon dioxide and methane adsorbed in carbon molecular sieve is studied at different compositions using a concentration-swing frequency response method and is described well by the new non-constant Fickian diffusion model. Results are compared with other predictive models.