A valve equation is incorporated into the simulation to consider dynamic change of the bed pressure and mass transfer between the gas phase and solid phase during the pressurization and blowdown steps of a simple two-bed air separation pressure swing adsorption (PSA) process. The valve equation approach is shown to provide a good representation of the cyclic steady state results and approach behavior of residual oxygen concentration in the product to cyclic steady state value, experimentally measured by Hassan ct al. (1986) for an air-carbon molecular sieve (CMS) system. The influence on product purity of the pressure-changing steps was also studied by this approach. For two PSA systems, air-CMS for producing nitrogen and air-5A zeolite for producing oxygen, the theoretical results obtained with the valve equation included are compared with those obtained from the assumption of a frozen solid phase and with the assumption of a linear pressure change.