Neutron reflectometry (NR) has been employed to study the structure and composition of thin films formed by 4-pentadecyl-pyridine (C15-4Py) at a gold electrode surface. The thickness and the water content of films of C15-4Py have been measured as a function of the potential applied to the electrode. At positive potentials, where condensed film is formed, the monolayer contains defects that are filled with water. At very negative potentials, the film is desorbed from the electrode surface. NR has demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface as a thicker and water rich film. (C) 2003 Elsevier B.V. All rights reserved.