We propose a complementary pressure swing adsorption (CPSA) process consisting of four beds for separating air into enriched oxygen and nitrogen. Simulation results showed that the CPSA process performed better than either the oxygen process or the nitrogen process. Using zeolite 5A and 4A as adsorbents, the mole fraction of oxygen and argon in the enriched oxygen may be as high as 99.57% (at 34.44% recovery), and the purity of the enriched nitrogen about 98% (at 47.86% recovery). For producing enriched oxygen of 80-90% purity, a modified CPSA process is suggested. The relationships between production rate, purity, and oxygen and nitrogen recoveries are discussed