This article describes an electrochemical metal-ion sensor based on a cobalt phthalocyanine (CoPc) complex and determination of its sensor activity for some transition metal ions. Ag+ and Hg2+, among several transition metal ions, coordinate to the sulfur donors of CoPc and alter the electrochemical responses of CoPc in solution, indicating possible application of the complex as Ag+ and Hg2+ sensor. For practical application, CoPc was encapsulated into a polymeric cation exchange membrane, Nafion, on a glassy carbon electrode and used as an electrochemical coordination element. This composite electrode was potentiometrically optimized and potentiometrically and amperometrically characterized as transition metal-ion sensors with respect to reproducibility, repeatability, stability, selectivity, linear concentration range, and sensitivity. A mu mol dm(-3) sensitivity of the CoPc-based sensor indicates its possible practical application for the determination of Ag+ and Hg2+ in waste water samples.