Steady state diffusion flux through a membrane is described considering non-equilibrium sorption-desorption process at the interfaces. A dimensionless parameter alpha (similar to second Damkoler number) is defined, which might serve as a measure for the deviation of the surface reactions from equilibrium. It is shown that alpha does not depend on the forward reaction rate constant. For alpha much less than 1, the surface reactions might be considered in 'near equilibrium'. For alpha comparable to or greater than unity, the permeability will appear dependent on the membrane thickness, and this dependence can be used to determine the surface reaction constants.