Silica thin films were first deposited on a copper substrate by potentiostatic cathodic electrolysis of a solution of ammonium hexafluorosilicate at 298 K. The film deposition according to the reduction of H+ was monitored by a quartz crystal microbalance and measured by X-ray fluorescence spectrometry. Characterization of the films by X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and scanning electron microscopy revealed that the film mainly consisted of SiO2 with silanol groups covering the surface. Basic reactions for film deposition are proposed including a pH-shift triggered by H-2 evolution. (C) 2002 The Electrochemical Society.