The oxygen evolution efficiency in seawater electrolysis has been examined for manganese oxide and manganese-tungsten oxide electrodes anodically deposited on iridium oxide-coated titanium substrates as a function of pH of the deposition electrolytes and tungsten content in the oxides. The oxygen evolution efficiency in 0.5 M NaCl solution at 30 degrees C On the MnO2 electrodes increases with a decrease in pH of the deposition electrolyte. The tungsten-containing electrodes show significantly higher efficiency for oxygen evolution. The tungsten addition enhances oxygen evolution and suppresses chlorine evolution, and hence 99.6% efficiency for oxygen evolution was obtained at 200 A m(-2) on the oxide with the molar ratio W6+/(Mn4+ + W6+) of 0.16. The anodically deposited manganese oxide consists of nanocrystalline gamma-MnO2. The manganese-tungsten oxides are also composed of a single orthorhombic nanocrystalline gamma MnO2-type phase, in which Mn4+ and W6+ ions are homogeneously distributed, even when cationic fraction of tungsten in the oxide is 0.16. The Tafel slopes of the MnO2 and (Mn-W)O-X electrodes for oxygen evolution are both higher than those for chlorine evolution. The MnO2 and (Mn-W)O-X electrodes anodically deposited at lower pH and the (Mn-W)O-X electrodes with higher tungsten content have smaller grain size and higher surface roughness. The surface roughening of the oxides leading to the decrease in the electrode potential under the galvanostatic condition accelerates the oxygen evolution reaction and suppresses chlorine evolution.