Electrochemical behaviour of electroless ZnS coatings on glass carbon and their interaction with Cu + and Cu 2 + ions were investigated by cyclic voltammetry in 0.1 M NaOH, 0.1 M KClO 4 and 1.0 M CH 3 COOH solutions. Independently of the medium pH cathodic reduction of ZnS was found to be preceded by hydrogen evolution. When ZnS interacts with Cu 2 + ions, non-stoichiometric copper sulphide (Cu 2 - x S, where 1.4 ≤ 2 - x ≤ 2) and elementary sulphur are formed according to the reaction (2 - x)ZnS + (2 - x)Cu 2 + → Cu 2 - x S + (2 - x)Zn 2 + + (1 - x)S 0 . The interaction of ZnS with Cu + proceeds with the formation of stoichiometric copper sulphide (Cu 2 S).