The permeability data of oxygen and nitrogen were obtained from both air and individual gas permeation experiments under different pressures using dense polyphenylene oxide (PPO) membranes of different thicknesses, after the membranes were fully stabilized. Opposite trends were observed with respect to oxygen and nitrogen as their permeabilities changed with changes in feed pressure and membrane thickness. Moreover, the pressure effect on oxygen permeability was reversed when the membrane was flipped upside down. Attempts were made to interpret the above experimental observations within a framework of entropy and enthalpy effect on the energy barrier for gas permeation and the change in the void space in the cross-sectional direction of a dense PPO membrane.