The electrochemistry of Mo(VI) in sulfuric acid solutions has been investigated at mercury and glassy carbon surfaces, the adsorption of Mo(VI) and electrogenerated products being evident at mercury electrodes. The adsorption phenomenon severely complicates the electrochemical behavior and depressed voltammograms and distorted chronoamperometric curves are obtained at relatively high Mo(VI) concentrations (> 1 mM). The influence of experimental parameters on the adsorption of molybdenum species onto the mercury surface such as electrode potential, acidity and time of adsorption was studied by measuring the amount of molybdenum by atomic absorption spectrophotometry. The catalytic process observed when nitrate or chlorate are added to the supporting electrolyte containing Mo(VI) does not appear when glassy carbon is used as working electrode. A dimeric Mo(V) species is formed when potentiostatic coulometric experiments are performed using mercury as a substrate. On the other hand, a black solid constituted by a nonstoichiometric molybdenum compound is deposited at the glassy carbon surface in similar experiments. The concomitant predissociation of molecular hydrogen during the cathodic reduction of Mo(VI) at glassy carbon electrodes, followed by migration of H atoms into the MoO 3 lattice, is responsible for the deposit formation.