A simple apparatus using the pressure-swing frequency response method has been extended to measure multicomponent diffusivities. Transfer phenomena are investigated for different compositions of a binary mixture of nitrogen and oxygen in a carbon molecular sieve. The paper has two objectives. First, we consider the applicability of some predictive multicomponent diffusion models, which predict the mixture diffusivities based on pure-component information. The results show that these models can provide qualitative but not quantitative descriptions. Second, we determine mixture diffusivities, including Fickian diffusivities and Maxwell-Stefan surface diffusivities, from the data using a new mathematical model. These diffusivities are concentration-dependent. A simple relationship that introduces a thermodynamic factor is used to describe the concentration dependence of the Fickian diffusivities. It reduces to Darken's equation for pure components. This approach, based on nonconstant Fickian diffusivities, provides an excellent description for all of the experimental data. An alternative approach is also developed for obtaining the Maxwell-Stefan surface diffusivities without using the empirical Vignes relationship.