The electrochemical stability of polycrystalline thin film MoxN (x = 1 and 2) electrodes deposited on previously nitrided polycrystalline Ti and bare Ti substrates and exposed to 4.4 M H2SO4 electrolyte has been investigated. The electrodes were prepared by a temperature programmed procedure involving the reaction of MoO3 films and NH3. Cyclic voltammetry and a.c. impedance spectroscopy indicated that the range of electrochemical voltage stability in the electrolyte of the MoxN films on nitrided Ti substrates, referenced to a standard hydrogen electrode to be -0.01 to + 0.69 V. These electrodes had a calculated capacitance of about 315 F cm(-3). The MoxN films on Ti substrates were electrochemically stable from -0.01 to + 0.67 V; however, they contained unconverted MoO2 which acted to reduce the calculated capacitance to about 210 F cm(-3). At and above + 0.70 V and + 0.68 V on nitrided Ti and Ti substrates, respectively, an electrochemical reaction occurred. The formation of an amorphous phase in the reacted MoxN films was indicated by X-ray diffraction (XRD). Secondary ion mass spectroscopy (SIMS) showed increases in the atomic concentrations of hydrogen, oxygen and sulfur in the reacted films. The voltage bias applied to the electrodes influenced the chemical composition of the reacted films.