The performance of a polyimide membrane module was investigated for the separation of air/CFC-12 mixtures as a function of temperature, pressure, stage cut, and feed gas composition. The ideal separation factor was in the range of 60 to 200 in the ranges of temperature and pressure studied. There was a substantial decrease in the permeation rates of the more permeable gas (air) through the glassy polymer (polyimide) caused by the presence of the other gas (CFC-12). However, the permeation rate of CFC-12 (dichlorodifluoromethane, CCl2F2), which is the less permeable gas, was increased by the presence of the more permeable gas (air). It was found that experimental data were well correlated with prediction of mathematical models.