The electrochemical oxidation of Mn(II) and Co(II) and reduction of Mn(III) and Co(III) at Pt in glacial acetic acid has been investigated by the rotating disc electrode technique. The diffusion coefficients of Mn(III), Mn(II), Co(III) and Co(II) ions, formal potentials and kinetic parameters of the Mn(III)/Mn(II) and Co(III)/Co(II) electrode reactions were evaluated. It was shown that the anodic oxidation of Mn(II) and Co(II) occurs at the surface blocked by a layer of adsorbed acetate ions. In addition, some experiments with an electrochemical quartz crystal microbalance (EQCM) were performed to explain the acetate adsorption on the surface of the polycrystalline platinum electrode.