Raman spectra of amorphous hydrated ruthenium oxide (RuO2 center dot xH(2)O) thin films were measured in situ during electrochemical charging/discharging cycles. As hydrogen was intercalated into the films, a peak at 590 cm(-1) emerged. The emergence of this peak was accompanied by a decrease of a peak at 700 cm(-1). To investigate the origins of the Raman peaks, isotope effects were studied using deuterated samples. Most of the Raman peaks showed no shift due to the heavier mass of deuterium, indicating that these peaks do not come from vibrations that involve hydrogen ions. Based on the changes in the Raman spectra, we conclude that the major effect upon intercalation of hydrogen is reduction of Ru4+ ions to Ru3+, and not creations of new chemical bonds. (C) 2005 The Electrochemical Society.