A compact non-stoichiometric molybdenum (VI, V) oxide of blue film was grown on carbon fiber (CF) microelectrode surface by cycling the potential between +0.2V and -0.70V in a freshly prepared Na2MoO4 solution containing 5×10^-^3mol/L H2SO4. The quantity of the oxide is controlled by the charge passing the electrode. The electrochemical pretreatment of CF microelectrode not only raises the deposition velocity of molybdenum oxide on CF surface, but also improves greatly the cyclic voltammetric behavior of the molybdenum oxide film prior to the electrodeposition. The cathodic processes are believed to yield the hydrogen molybdenum oxide bronzes, HxMoO3 (0

Key words: X-RAY PHOTOELECTRON SPECTROMETRY, SCANNING ELECTRON MICROSCOPES, CHEMICAL MODIFIED ELECTRODE, MOLYBDENUM OXIDE, STOICHIOMETRY, MICROELECTRODE