Integrally-skinned asymmetric polysulfone (PSF) membranes were surface fluorinated under mild treatment conditions, according to one of two strategies : at fluorination times of 1 to 10 min with a constant fluorine feed concentration of 0.02% F2 (by volume), or at concentrations ranging from 0.02 to 0.15% F2 for a fixed treatment time of 2 min. The fluorinated region was characterized using dynamic contact angle analysis and X-ray photoelectron spectroscopy. It was shown that increasing fluorination times from 1 to 5 min resulted in increased F/C and O/C ratios in the fluorinated subsurface region and also increased the polar surface energy contribution (gamma(s)p) at the surface. Longer fluorination times resulted in an apparent decrease in polarity, possibly due to surface damage. For increasing fluorine concentrations at a constant treatment time, the F/C and O/C atomic ratios in the subsurface region increased up to 0.04% F2 and effectively leveled off at higher feed concentrations. At the surface, gamma(s)p decreased after reaching a maximum at 0.06% F2. For both treatment strategies the ideal selectivities for certain gas pairs (He or H-2 with N2 or CH4) follow trends similar to that of gamma(s)p. Decreases in selectivity appear to be associated with fluorination damage (surface roughness and damage to the polymer structure), rather than with a change in the chemical nature of the fluorinated skin region.