Surface transport is described in terms of the hopping of adsorbed molecules between adjacent sites of different adsorption strength. The change in surface diffusivity with surface concentration is attributed to a change in the strength of adsorption, as evidenced by a change in the differential heat of adsorption, q, with concentration. The correlating equation predicts that the surface diffusivity varies as exp(~aq/RT), where a is an experimental constant. New data on the flow of C02, S02, and NH3 in porous glass are presented and are correlated successfully by the above equation. In addition, literature data for five other systems are correlated well by this method.