A study of CO oxidation by O-2 over Pt catalysts, promoted by MnOx and CoOx, is described. The activities of Pt/SiO2, Pt/MnOx/SiO2 and Pt/CoOx/SiO2 are compared with commercial Pt/Al2O3, Pt/Rh/Al2O3 and Pt/CeOx/Al2O3 catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found : Pt/CoOx/SiO2 > Pt/MnOx/SiO2, Pt/CeOx/Al2O3 > Pt/Al2O3, Pt/Rh/Al2O3, Pt/SiO2. Over Pt/CoOx/SiO2 CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoOx/SiO2 in the CO/O-2 reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoOx/SiO2, Pt/MnOx/SiO2 or Pt/CeOx/Al2O3 catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoOx/SiO2 catalyst in CO oxidation.