Recent measurements of the Onsager heat of transport Q* for ammonia at the surface of water have found Q* to be constant for gas pressures over the range 10-900 Torr. This result cannot be understood in terms of a molecular-scale model for the origin of the heat of transport at a gas-liquid interface, which predicts that Q* should decrease with increasing pressure, and must be regarded as a direct consequence of Onsager's reciprocal relation for coupled heat and matter fluxes. (C) 2009 Elsevier B.V. All rights reserved.