The aim of this study was to examine the saturation patterns of cuprofane and polysulfone membranes in hemodialysis patients, and to correlate that with permeability of the alveolar membrane, unloading of CO2over the membranes, and production of oxygen free radicals. Polysulfone membranes did not have the same desaturation pattern as the cuprofane membranes. Both membranes were equally effective in the unloading of CO2, and so alveolar hypoventilation does not explain oxygen desaturation during hemodialysis. White blood cells have a central role in the parallel development of early leucopenia and hypoxemia. The precise mechanism is still unknown because there were no changes in permeability of the alveolar membrane or production of oxygen free radicals.