The Onsager heat of transport Q* at the liquid-vapour interface of water is constant at -24.3 +/- 3.6 kJ mol(-1) (two standard deviations) over the pressure range from 4 to 9.5 Torr. This result is consistent with a model in which Q* is determined by the temperature gradient across the Knudsen layer adjacent to the liquid, but not with one based on perturbed temperature profiles from temperature-jump theory, which predicts that the measured heat of transport should vary inversely with pressure. It implies that the Onsager heat of transport should be significant for gas-liquid exchange processes even at atmospheric pressure. (c) 2006 Elsevier B.V. All rights reserved.