The effect of a surface temperature gradient on the water vapour pressure over aqueous sulfuric acid has been measured for acid concentrations in the range from 33% to 80% by weight. Graphs of Delta P versus Delta T are linear, for both positive and negative Delta T, at thicknesses of the vapour layer up to at least 10 mean free paths. At high acid concentrations, the rate of evaporation is limited by the rate of dissociation of sulfuric acid monohydrate, with the result that the effect of the temperature gradient, and hence the measured heat of transport, is greatly reduced. (c) 2005 Elsevier B.V. All rights reserved.