Pre-adsorbed and bulk (continuous) CO oxidation on a polycrystalline Pt electrode were examined in a wall-jet electrochemical quartz crystal nanobalance (EQCN) setup, using both differential and integral evaluation of the EQCN data, to get further insights into the kinetics and mechanism of this important fuel-cell related electrocatalytic reaction. The hydrogen underpotential adsorption-desorption features in the base cyclic voltammogram of a Pt film are accompanied by significant changes in the electrode mass due H-upd induced desorption-adsorption of anion. In the double-layer region small capacitive currents are accompanied by comparatively large reversible mass changes indicating anion adsorption/desorption (96.5 g mol(-1) assigned to bisulfate). OH and oxygen electrosorption from water at potentials more positive of 1.0 V result in relatively small variations in the electrode mass (16 g mol(-1) for PtOH and ca. 9 g mol(-1) for PtO formation, respectively). The CO-adlayer stripping first leads to the electrode mass decrease in the '' pre-peak '' region, followed by a fast mass increase within the main stripping peak due to re-adsorption of bisulfate anion (91 g mol(-1)). A mass-transport limited current for bulk CO oxidation under continuous flow of CO-saturated electrolyte leads to negligible mass changes (0-1 g mol(-1)) in the PtO region, suggesting that bulk CO oxidation is mediated by electroformed PtO. (c) 2006 Elsevier B.V. All rights reserved.