The Onsager heat of transport at a gas-liquid interface is obtained as a consequence of the requirement of detailed balance for the fluxes of gas molecules arriving at and departing from the liquid surface, together with the assumption that the effect of the temperature gradient on the free energy barrier for evaporation is given by DeltaG(vap)(1) = DeltaG(vap) - etaDeltaTDeltaS(vap), where DeltaT is the temperature, difference across a thin layer of vapour adjacent to the surface of the liquid and eta is a transfer coefficient. (C) 2004 Elsevier B.V. All rights reserved.