We have calculated the potential of mean force (PMF) for the transfer of a solute molecule across a liquid vapour interface for four different systems: (a) one methanol molecule in water, (b) one water molecule in methanol, (c) one acetonitrile molecule in water and (d) one water molecule in acetonitrile by means of constrained molecular dynamics simulations. A minimum of the PMF is found near the Gibbs dividing surface for methanol and acetonitrile solutes although the degree of surface activity is found to be somewhat different due, in part, to varying hydrogen bonding nature of these two solutes. (C) 2004 Elsevier B.V. All rights reserved.