Asymmetric and slightly pressure dependent permeation properties of commercial air separation hollow fibers have been observed - phenomena that have not been mentioned in the literature before. In bore-side feed the permeance of oxygen, nitrogen and helium slightly increases with pressure while in shell-side feed the permeance decreases with pressure. At a feed pressure of 12.8 bar(a) the permeance in bore-side feed is about 10-15% higher than in shell-side feed mode. Interestingly, the effect was more or less similar for different gases and different fibers. The results suggest that mechanical stress induced by the pressure difference across the membrane affects the permeability of the skin layer.