Abstract The effect of the specific adsorption of the ClO 4 − ion upon the kinetics of Cu 2+ reduction and Cu 0(Hg) oxidation in aqueous solutions of HClO 4 was investigated using chronocoulometry, d.c., normal pulse, and a.c. polarography. For this purpose three different sets of data for ClO 4 − adsorption were used. In the first set the absolute surface excess of water as reckoned at the outer Helmholtz plane (o.H.p.) was disregarded, in the second set it was accounted for by postulating a complete adsorbed monolayer of water molecules , whereas in the third set partial displacement of water molecules from this monolayer by adsorbed ClO 4 − ions was accounted for. Kinetic data were found to the compatible with the latter set, indicating that ClO 4 − adsorption affects the rate-determining step Cu 2++ e→Cu + only by altering the average electric potential σ d-effect). The absence of any additional electrostatic effect points out that, in the transition state for Cu 2+ reduction to Cu +, the centre of charge of the reacting particle falls in the proximity of the o.H.p.