Adsorption of sulfate ions on the three basal planes of monocrystalline copper electrodes in 0.1 M HClO4 was studied by radiometric and electrochemical methods. Also, the copper electrode activity for hydrogen ion discharge was investigated. To check the order of copper surfaces the underpotential deposition of lead was applied. Adsorption of sulfate ions on all four surfaces was found to be reversible with respect to the potential and bulk solution concentration of sulfate. Adsorption isotherms were determined radiometrically. The Gibbs energy of adsorption was calculated by fitting the Henry, Frumkin and virial isotherm equations to the experimental data points. The limiting surface concentration of sulfate ions decreases in the sequence: Cu(111) > Cu(110) > Cu(poly) > Cu(100). The highest value of surface concentration of sulfate for the (111) plane can be accounted for by a match between the tetrahedral structure of the anion and the trigonal distribution pattern of the surface atoms of copper on this plane.