The adsorption and electrocatalytic oxidation of carbon monoxide on platinum was studied in both acid and alkaline supporting electrolytes using an electrochemical quartz crystal microbalance. Depending on the pH of the medium, a significant influence of CO on the electrode mass was observed, particularly in the potential range preceding the complete oxidation of CO. In acid medium coupling with solvent molecules was also evidenced and was found to be potential dependent. Furthermore, the mass responses depended strongly on the way the adsorption of CO was realised. In addition, the effect of the bulk concentration of CO was investigated in the case of perchloric acid medium.