Characteristics of air separation are determined in a serial configuration of hollow liber polysulfone membranes. One, two, and three separation cells in series are used in the measurements. All systems are operated in the counter-current flow mode and effects of the reject flow rate and feed pressure are considered in the measurements. The plug flow model is used to simulate and analyze the system. Results include variations in species permeance, stage cut, permeate enrichment, reject depletion, and recovery of oxygen and nitrogen gases. Most of the plug flow model predictions are found to closely match the measured data, with deviations less than 10%. However, deviations in N-2 recoveries are found to be larger than other system parameters, with deviations close to 30%. increase of the number of separation cells results in higher stage cuts and in turn to higher species recovery in the permeate stream. Simultaneously, the purity of the reject is increased and that of the permeate stream is decreased. At constant reject flow rate, the highest permeate enrichment is found in the permeate stream of the first cell in the two- and three-cell systems. This is caused by the increase in the feed flow rate, which results in reduction of the gas residence time and in turn the gas permeation is highly selective and is dominated by the fast permeating species O-2.