The flux equation for gas transport through a glassy polymer membrane is expressed in terms of a chemical potential gradient. It is an implicit function of the operating pressure. By studying the specific properties of this function, it can be simplified to an explicit form, which is mathematically identical to the expression proposed by the dual mobility model. Using this new model, all the data obtained by the dual mobility model can be described adequately. However, the physical significance of the parameters of the present model is completely different from the parameters of a dual mobility model. The present model assumes that only the molecules sorbed by the Henry mode contributed to the flux, and those sorbed by the Langmuir mode remained completely immobilized. This model provides an indirect method for the determination of partial molar volumes of gases in polymer membranes. It has been shown that the steady-state flux depends on the sorption rate at the interface, and not on the Henry constant. It has been found that the contribution of pressure in the diffusional transport of gases is of the same order and may be even higher than that of the concentration, which contradicts the popular perception that 'the diffusion process is related basically to the concentration gradient'.