The adsorption of equimolar binary Lennard-Jones gas mixtures into single-walled carbon nanotubes is investigated using grand canonical Monte Carlo simulations. For mixtures whose species have different molecular diameters, the larger energetically favored species is adsorbed at higher temperatures. However, at. lower temperatures and intermediate nanotube diameters, a complete exclusion of the larger species in favor of the smaller species is observed. This transition in nanotube fluid composition is accompanied by a decrease in the total potential energy of the system. Although adsorption of the smaller species is favored, both species adsorb at lower temperatures in the larger nanotubes. In situations where the molecular diameters are similar, the energetically favored species is preferentially adsorbed at all temperatures. Axial pair correlations are used to relate the composition in the nanotube with the structure of the adsorbed fluid.