Electrochemical behaviour of electroless ZnS coatings on glass carbon and their interaction with Cu+ and CU2+ ions were investigated by cyclic voltammetry in 0.1 M NaOH, 0.1 M KClO4 and 1.0 M CH3COOH solutions. Independently of the medium pH cathodic reduction of ZnS was found to be preceded by hydrogen evolution. When ZnS interacts with Cu2+ ions, non-stoichiometric copper sulphide (Cu2-xS, where 1.4 less than or equal to 2-x less than or equal to 2) and elementary sulphur are formed according to the reaction (2-x)ZnS + (2-x)CU2+ --> Cu2-xS + (2-x)Zn2+ + (1-x)S-O. The interaction of ZnS with Cu+ proceeds with the formation of stoichiometric copper sulphide (Cu,S).