The effects of submonolayer amounts of metal ad-atoms, including Bi, Pb, Tl and Cd, on the electrochemical oxidation of formic acid and dextrose were studied on Pt and Au electrodes. Bi, Pb and Tl ad-atoms were found to greatly enhance the anodic currents obtained for the oxidation of the organic compounds of Pt. However, the metal ad-atoms were found to greatly decrease the anodic currents for the oxidation of dextrose on Au. The contrasting behavior of the metal ad-atoms was concluded to result from the fact that adsorbed "poisons" were formed on the Pt electrode during the electrochemical oxidation of dextrose, but not on the Au electrode;The temperature dependence of the anodic currents obtained during the electrochemical oxidation of formic acid was investigated on a Pt electrode for the presence and absence of Bi ad-atoms. The independence of the peak potentials as well as peak shapes on temperature was interpreted as a consequence of the high rate of the electron-transfer step. A mechanism for the enhancement effects observed was proposed which contains, as the rate-limiting step, the desorption of poisons from the electrode surface. The metal ad-atoms were concluded to cause enhancement for the oxidation of dextrose and formic acid because they increase the frequency of desorption of the poisons.