A muVT-ensemble Orientational-Bias Monte Carlo technique has been utilized to simulate adsorption isotherms of pure and mixed propane/propylene in NaA zeolite at 298.15 K. An atomic potential model of Catlow et al. (1991) and Oie et al. (1981) is found to represent the experimental isotherms satisfactorily. The simulations reveal the reason why propylene is adsorbed much more strongly than propane; i.e. the adsorption energy of propylene is 10 kJ/mol larger than that of propane, mainly through the Coulomb interactions between propylene molecules and zeolite atoms/Na ions. Pure adsorption isotherms of propane/propylene obtained from the simulations are correlated with four theoretical models: Langmuir, multi-site occupancy, two-dimensional van der Waals fluid and Ruthven+U models. The latter three models are found to fit much better than the Langmuir model because they take account of interactions between adsorbed molecules, the contribution of which has been clarified to be significant from the simulations. The simulation isotherms of propane/propylene mixtures, calculated at 100 kPa in total pressure and 298.15 K, show a novel feature of mixed-gas adsorption; i.e. the adsorption of propane, a weak adsorbate, increases with the decrease in the partial pressure of propane and reaches a maximum in a low mole fraction of proylene. This unusual mixed-gas adsorption has been ascribed to a "sorption effect" through the molecular interactions of propylene molecules appreciably adsorbed in zeolite. The simulation mixedgas isotherms are qualitatively well predicted by the above three models, while the Langmuir model with the IAST (Ideal Adsorbed Solution Theory) fails to predict the existence of a maximum adsorption of propane.