For condensation of binary mixtures where the less volatile constituent has the higher surface tension the condensate film is potentially unstable and various "modes" of condensation are seen depending primarily on composition and vapour-to-surface temperature difference. Of particular interest is the so-called "pseudo-dropwise" mode of condensation where the appearance of the condensate closely resembles that of dropwise condensation of a pure fluid on a hydrophobic surface. In recent years the so-called Marangoni condensation problem has been studied experimentally in great detail by Utaka and co-workers [4-8,11] for condensation of steam-ethanol mixtures on small vertical plane surfaces. It has been found that very small concentrations of ethanol in the liquid phase can give rise to significantly larger heat-transfer coefficients than found with pure steam. In the present investigation the problem has been studied using a horizontal condenser tube. Heat flux and vapour-to-surface temperature difference have been measured for steam-ethanol mixtures over a wide range of composition at atmospheric pressure. The results show the same trends as those found by Utaka for vertical surfaces. Differences in detail can be explained by geometry considerations and strong dependence of heat-transfer coefficient on vapour-to-surface temperature difference and vapour velocity, both of which vary around the perimeter of the horizontal tube. (c) 2007 Elsevier Ltd. All rights reserved.