The electrochemical and adsorptive behavior of formaldehyde at Pt electrodes in acidic media was investigated using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques. All chemical and electrochemical steps related to formaldehyde oxidation (e.g. bulk adsorption and oxidation, CO (sub)monolayer adsorption and oxidation and electrons per Pt site) were analyzed. All the mass and charge density data in this paper are referred to the real surface area. The charge density associated with formaldehyde oxidation was close to 420 muC cm(-2). which is related to the oxidation of approximately one CO monolayer with two electrons transferred. For CO adsorption the experimental mass value was 50 ng cm(-2). In the region of CO oxidation the analysis of mass and charge variations indicates simultaneous CO oxidation, anion and water adsorption and CO readsorption. The mechanism was confirmed by CO and CO2 flux calculations. From the analysis of the mass-charge ratio and species flux it was concluded that CO, an intermediate produced during formaldehyde oxidation, is adsorbed at the Pt surface and the main contribution to the mass increase during formaldehyde oxidation is CO readsorption, and water adsorption. (C) 2004 Elsevier Ltd. All rights reserved.