The adsorption behaviour of Pt was investigated with the electrochemical quartz microbalance during potential scanning in acid electrolytes. From the mass increase with potential it is concluded that in the potential range of hydrogen desorption and double layer both water and anions are adsorbed in H2SO4 and H3PO4, whereas in HClO4 there is only adsorption of water molecules. The mass changes in the oxygen region indicate that anions are included in the oxide layer. The Pt/COads system was studied in H2SO4, HClO4 and H3PO4. The presence of adsorbed CO is observed as a mass increase with respect to the CO-free system. The results show that water is adsorbed on top of the adsorbed CO layer at low potentials. Moreover, it can be concluded from the mass-balance data that water molecules are attached to the hydrogen monolayer at 0.03 V vs R.H.E. in sulfuric and perchloric acid, but not in phosphoric acid.