In this study, we used constrained molecular dynamics techniques to investigate the transport of a hydronium ion across the water liquid/vapor interface. The computed transfer free energy was nearly unchanged as the hydronium ion approached the Gibbs dividing surface. The ion crossed the interface with no substantial minimum free energy, and transport of the hydronium ion involved a change in the solvent composition of the solvation shells around the ion. (C) 2003 American Institute of Physics.