Constrained molecular dynamics techniques were used to investigate the mechanism of the transfer of water and ammonia molecules across the liquid/vapor interface of water. The computed potentials of mean force were nearly constant when the solute was more than several angstrom from the Gibbs dividing surface and decreased with no substantial minimum free energy as they crossed the liquid/vapor interface. The computed solvation free energy for water, estimated from the potential of mean force, was in excellent agreement with the experimental measurement while the corresponding computed solvation free energy for the ammonia molecule somewhat over estimated the experimental value. (C) 2004 Elsevier B.V. All rights reserved.