Transport of binary O-2/Ar gas mixtures in a carbon molecular sieve (CMS) has been studied using concentration-swing frequency response. The rate data for this binary gas system are described well by the transport models (surface barrier for Ar and surface barrier plus micropore diffusion for O-2) and parameters that were found to govern transport of the respective pure gases on the same CMS. These results suggest that transport of O-2 is not impeded by the presence of Ar. However, the experimental data do not rule out the possibility of some enhancement of the O-2 transport rate. Furthermore, O-2 and Ar isotherms on this CMS were observed to be linear in the region over which frequency response experiments were performed, suggesting that the micropore diffusion component of O-2 transport behavior is unaffected by the presence of Ar. Because transport of Ar in this CMS is much slower than O-2, the dynamic response of this system owes the majority of its character to O-2 adsorption, and the effect of O-2 on Ar transport remains hidden.