A compact blue conducting mixed-valence Mo (VI,V) oxide film was grown on the surface of a carbon fibre (CF) microelectrode by cycling the potential between +0.20 and similar to 0.70 V SCE in freshly prepared Na2MoO4 solution in H2SO4 (pH 2). The thickness of the oxide film was controlled by the charge passed. X-ray photoelectron spectroscopy (XPS) examination confirmed that the components of the molybdenum oxide film on the CF electrode surface are mainly a mixture of Mo6+ and Mo5+ complex, together with a small amount of Mo4+; no Mo3+ peak was observed. Well-defined redox transitions have been attributed to the reductive formation of hydrogen molybdenum bronzes [MoO3-x (OH),] and re-oxidation in acidic media. The coating is easily permeable to ions. The molybdenum oxide film catalyses the electroreduction of chlorate to chloride and bromate to bromide in H2SO4. At a bare CF microelectrode, chlorate and bromate are not reduced prior to the onset of hydrogen evolution at about -1.0 V. At the modified surface, a chlorate or bromate reduction peak appears at about -0.4 V SCE. The peak current is linear with concentration from 10(-5) to 10(-3) M chlorate or from 10(-6) to 10(-4) M bromate, and is nearly diffusion controlled. The electrocatalytic activity of H0.34MoO3 towards chlorate or bromate ions is even stronger than that of H0.93MoO3. The reactivity sequence determined by comparing current magnitudes on the molybdenum oxide film modified CF microelectrode is BrO3- > ClO3-.