Sb adsorption and coadsorption with CO on Pt electrode in perchloric acid solutions were studied by using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The results demonstrated that Sb adatoms are stable on Pt electrode in the potential range between - 0.25 and 0.50 V (SCE), and that the saturation coverage of Sb is 0.34. Data obtained from the studies of EQCM and programmed cyclic voltammetry shows the number of electrons transferred in oxidation of Sb-ad on Pt electrode in a positive going potential scan till 0.50 V has been calculated at 3.02 +/-0.06, and the Sb(OH), species were suggested as the adsorbed products of Sb-ad oxidation. At potential higher than 0.50 V the Sb(III) can be oxidized further and will desorb progressively from Pt electrode surface. It has been demonstrated that the oxidation of adsorbed and solution CO species on Pt and Pt/Sb-ad electrodes undergoes different surface processes. The electrocatalytic effect of Sb-ad on the oxidation of COad consists mainly of the negatively shift of oxidation potential, i.e. the activation energy of COad oxidation is decreased on a Pt/Sb-ad electrode in comparison to the same reaction carrying out on a Pt electrode.