Manganese-molybdenum oxide electrodes were prepared by anodic deposition on an IrO2-coated titanium substrate at a constant current density of 600 A m(-2) from baths containing 0.2 M MnSO4 and 0-0.1 M Na2MoO4 at 90 degrees C and pH 0.5. These electrodes were characterised for oxygen evolution in the electrolysis at 1,000 A m(-2) in 0.5 M NaCl solution at 30 degrees C and pH 8 or 12. The most active and stable oxygen evolving anode exhibited 100% efficiency for oxygen evolution, and an efficiency of 98.5% for over 1,500 h at pH 12 and of 96.5% for over 2,800 h at pH 8 of continuous electrolysis. X-ray diffraction measurement and XPS analysis indicated that the deposits consist of a nanocrystalline single gamma-MnO2 type phase, and manganese and molybdenum in the deposits are in the Mn4+ and Mo6+ states. The electrochemical studies showed that the manganese-molybdenum oxide electrodes drastically reduced the electrocatalytic activity for chlorine evolution to the undetectable level, resulting in 100% efficiency for oxygen evolution, although the addition of molybdenum slightly increased the oxygen overpotential.