The importance of the cation effect on the electro-oxidation of glycerol and ethylene glycol on Pt and Au in alkaline media was investigated by cyclic voltammetry and chronoamperometry. The reaction on Pt is influenced by OHad-M+(H2O)x clusters from non-covalent interactions between hydrated alkali metal cations (M+(H2O)x, M+ = Li+, Na+ and K+) and OHad on Pt, as previously reported for many electro-catalytic reactions on Pt. Contrarily, the impact of OHad-M+(H2O)x clusters on the electro-oxidation of these alcohols over Au is not significant. Different behaviors of Pt and Au in alkaline solutions are not associated with distinct surface coverage degrees of oxygenated species formed on Pt and Au at the glycerol and ethylene glycol oxidation region, but rather with the potential region at which the electro-oxidation of these alcohols takes place on Pt and Au. The reactions occur at lower potentials on Pt than on Au. At high potentials, electrostatic repulsion between the positively charged Au surface and cations is mandatory for weakening non-covalent interactions between hydrated alkali metal cations and surface oxygenated species.