The solvation of charge at the interface between an aqueous phase and hydrophobic medium is a key question in the mechanism of a wide range of biomolecular processes. These studies report the direct measurement of the molecular properties of water surrounding an isolated charge at an oil/water interface, and show how the solvating water structure changes with progressive charge build-up. At trace charge concentrations, highly oriented solvating water molecules found in both the aqueous and hydrophobic rich phases have negligible interactions with other water molecules. Water molecules observed within the hydrophobic rich phase are highly oriented by charge-dipole interactions. (C) 2003 Elsevier B.V. All rights reserved.