A solid-state electrochemical method was applied to determine the forms of adsorbed oxygen species on the surface of catalysts by the theory of statistics and electrochemistry. The forms of adsorbed oxygen species on the platinum surface were demonstrated to play a significant role in the light-off property of CO oxidation. On the contrary, the forms of adsorbed oxygen species on CuO had no influence on CO oxidation. The adsorbed O2- ion predominantly formed on the platinum surface at 200 degrees C was suggested as a crucial form in the light-off phenomenon. A competitive backdonation model was proposed to reasonably illustrate the mechanism of the light-off phenomenon of CO oxidation over platinum catalysts.