Diffusion of copper in copper sulfides, Cu2-xS is studied as a function of x by means of electrochemical impedance spectroscopy (EIS) at room temperature. Copper sulfide compositions were determined from electromotive force determinations in the Cu\Cu(2+)aq\Cu2-xS galvanic cell. The same cell was used to change the x value by imposing a potential different of the equilibrium one. Experimental EIS determinations relate diffusion coefficients and concentrations of diffusing species. The experimental data are discussed accordingly. (C) 1939 The Electrochemical Society. S0013-4651(99)01-087-3. All rights reserved.