The experimental determination of mixed gas isotherms is challenging and thus rarely performed. Nevertheless, characterizing the performance of adsorbents toward mixtures of gases is critical in most adsorptive separations. Here, the utility of Raman spectroscopy in determining binary gas adsorption isotherms on the microscale with metal-organic framework (MOF) single crystals is demonstrated for quantifying C2H6/CH4 selectivity. The influence of pore size on sorption selectivity is determined experimentally. The technique also allows determination of kinetics of methane adsorption in MOFs, which is critical for refueling times in adsorbed natural gas storage.