Computer simulations have been carried out to study the performance of a two-bed PSA system for the separation of methane and nitrogen. The concentration of CH, in the low-pressure product has been assessed in terms of the adsorption pressure, CH, content in the feed gas and the cycle length. This concentration is found to exhibit a distinct maximum with respect to the pressure of the adsorption step. The occurrence of the maximum is attributed to selectivity varying with pressure, which, in turn, results from the nonlinearity of the CH4 and N-2 adsorption isotherms. If linear isotherms are assumed, as is the case in simplified solutions, no such maximum is observed and the model fails to predict the crucial characteristics of the system.