Flat sheet integral asymmetric polysulfone membranes were subjected to gas phase surface fluorination in a well-mixed reactor. Short residence times ensured a constant fluorine concentration in the reactor (i.e., at the membrane surface) after 1 min of operation, so that the fluorination time and the fluorine feed concentration could be investigated independently. Fluorination conditions were optimized to yield an improvement in the selectivity of the gas pairs O2/N2, H-2/N2, H-2/CH4, He/N2, He/CH4 and CO2/CH4 with a varying decrease in the permeability of all gases. For example, at 0.02% F2 the O2/N2 selectivity was improved by up to 50% at fluorination times of 2 min. However, 5 min of fluorination produced the optimum (more than ten-fold) increase in the He/CH4 selectivity with an average decrease in He permeance (P/l) of only 27%. Low fluorine feed concentrations of 0.02-0.04% gave optimum results and higher concentrations resulted in a sharp decline in the selectivity of all gas pairs. Good transport properties were also obtained by subjecting membranes to successive fluorination cycles of short duration at 0.02% F2. Two cycles of 1 min each produced optimum results. Membranes coated with highly permeable silicone rubber (PDMS) after fluorination invariably showed an increase in selectivity with a relatively small additional loss of permeability.