Pure gas adsorption isotherms of ethane and ethylene on 13X molecular sieves were determined at 25 and 50 °C. Data for the binary adsorption equilibria of these two gases were collected at the same temperatures and a pressure of 1033.9 mmHg (20 psia). With these data, models based on the following types of approaches have been evaluated: assuming the adsorbed phase is a thermodynamically ideal solution, assuming the adsorbed phase has a quasicrystalline structure (lattice solution model), treating the adsorbed phase as a two-dimensional gas, and applying simplified statistical thermodynamics to the adsorption process. For these systems the thermodynamically ideal adsorbed solution model gave the best results overall, while the two-dimensional gas model and the statistical thermodynamic approach also gave reasonably good predictions.