The initial stages of cobalt–molybdenum electrodeposition on a vitreous carbon electrode were studied to obtain information about the mechanism of cobalt–molybdenum induced codeposition. Solutions containing cobalt sulphate, sodium molybdate and sodium citrate at pH 6.6 were used. A first step in the mechanism of alloy deposition is proposed. This step takes into account the formation of molybdenum(IV) oxides over which Co–Mo alloy may be only deposited if sufficient potential is applied. Co–Mo electrodeposition occurs through an early stage involving low reduction current, related to the formation of molybdenum oxides, followed by a later stage in which the reduction current suddenly increases, corresponding to alloy codeposition. When a low potential is applied, a continuous coloured molybdenum oxide film is formed on the electrode and Co–Mo is not deposited. To induce the alloy deposition on the ‘oxide film’ it is necessary to apply more negative potentials than a threshold value, which depends on the composition of the electrolytic bath. By increasing molybdate concentration in solution, the ‘threshold potential’ shifts to more negative values. Intermediate molybdenum oxides were characterized using scanning electron microscopy (SEM), compositional analysis, Raman measurements and Auger and X-ray photoelectron spectroscopies.