A blue, conductive, compact, mixed-valent Mo(VI, V) oxide film was grown on carbon fiber (CF) microelectrode surface by cycling the potential between +0.20 and -0.70 V vs. SCE in a fresh prepared Na2MoO4 solution (pH = 2, H2SO4). The thickness of the oxide film was controlled by charge passing through. X-ray photoelectron energy spectra (XPS) confirmed that the products of molybdenum oxide film coating on the CF electrode surface are mainly a mixture of Mo6+ and Mo5+ complex, meanwhile a little of Mo4+ exists in the film, but the Mo3+ peak was not observed in XPS. Well-defined redox transitions have been attributed to the reductive formation of hydrogen molybdenum bronzes [MoO3-x(OH)x] and reoxidation in acidic media. The coating is easily permeable to ions. The molybdenum oxide film catalyzes the electroreduction of chlorate to chloride and bromate to bromide in H2SO4.