Asymmetric polysulfone membranes were prepared by the phase inversion technique under different solvent evaporation conditions prior to the gelation step. The membranes were cast from the two component system of polymer and N,N-dimethylacetamide in which the polymer concentration was changed from 25.0 to 30.0%. The solvent evaporation temperature was changed from 70 to 120 degrees C, and the evaporation time was 0-15 min. Ethanol, water, or 2-propanol was used as the gelation media. The membranes were characterized by the measurement of oxygen/nitrogen permeation with the lamination technique and by observation with scanning electron microscopy. With an increase in the solvent evaporation time, the oxygen permeance decreased and its selectivity over nitrogen increased; although the permeance was in the range of 1-2 GPU, the oxygen selectivity over nitrogen exceeded 8. A correlation between the permeation performance and the operational parameters involved in the solvent evaporation process was obtained.