A new copper(II) ion selective microelectrode is presented. The sensor is based on a copper sulfide of complex composition. The crystalline sensor material is embedded in a polymer membrane which is deposited on a microdisc platinum electrode. The electrode response gives an unusual linear slope of -50 mV/p Cu2+ over a concentration range of 1.0 to 10(-4) M Cu2+ with a detection limit of about 10(-6) M in a CuSO4 solution of pH 4.0. A model to explain the slope of -50 mV/p Cu2+ is proposed The sensor is characterized electrochemically, by X-ray photoelectron spectroscopy, ICP, X-ray powder diffraction and SEM pictures. Some applications of the electrode are presented.