Separation characteristics of CO2, O-2, and N-2 gas mixtures are measured in a series configuration of hollow fibre polysulfone membranes. One, two, and three separation cells in series are used in the experiments and are operated in the counter current mode. The measurements are performed as a function of the reject flow rate and pressure. Measured data are compared against predictions of the countercurrent flow model. Measurements and analyses include variations in the stage cut as well as species permeance, removal, recovery, permeate enrichment, and reject depletion. Measurements and model predictions are closely matched with percentage errors of less than 10%. Operation with three cells in series results in larger stage cuts and higher species recovery in the permeate stream. This is associated with lower purity in the permeate and reject streams. The highest permeate enrichment is found for the permeate stream in the first cell for the two and three cells system. This is caused by the constant reject flow used in various configurations. As a result, higher feed flow rates are used for the two and three cells in series, which reduces the gas residence time in the first cell and makes separation more selective.