The co-catalytic effect of Mo on the oxidation of adsorbed CO is examined on different types of electrodes using differential electrochemical mass spectrometry (DEMS). Mo surface composition is determined by XPS. Mo was either deposited electrochemically onto Pt(111), Pt(332), smooth and porous polycrystalline Pt electrodes, or prepared by co-sputter deposition. From a comparison of the surface coverage of Mo on Pt(111) and the corresponding oxidation charge, a number of six electrons per adsorbed Mo is estimated, meaning that below 0.2 V adsorbed Mo has a formal oxidation number of zero. Since there is a linear relationship between the oxidation charge of Mo and the suppression of CO adsorption when varying the Mo coverage, we conclude that CO does not adsorb on Mo. From the oxidation charge and the number of CO adsorption sites suppressed, we obtain a number of three electrons per site; therefore, one Mo atom blocks approximately two Pt sites. DEMS reveals, that CO2 formation starts around 0.15 V, overlapping with the onset of Mo oxidation. The action of Mo is to shift the usual prepeak (weakly adsorbed state) from 0.5 to 0.3 V, whereas the main oxidation peak (strongly adsorbed state) is hardly affected. This resembles the effect of Sn; however, Sn forces up to 50% of the adsorbed CO from the strongly adsorbed state into the weakly adsorbed state, independent of the Sn coverage, whereas in the case of Mo, only around one tenth of the adsorbed CO is oxidized in the prepeak, independent of Mo coverage. Therefore, we explain the effect of Mo by an oxygen spillover effect, which is only working for CO in the weakly adsorbed state, and not by an electronic, repulsive interaction as in the case of Sri. With sequential step decoration on Pt(332) by Ru and Mo, it is shown that a synergetic effect of both co-catalysts is obtained. The effect of Mo on the oxidation of COad formed by adsorbing methanol and on the oxidation of methanol is very small.