A sensor has been developed for measurement of high concentrations of copper in molten phases. Cusicon (CuZr2(PO)(4)) electrolyte was used and instead of measuring the potential across the electrolyte from the reference material to copper containing phase, a potential was applied and the current measured. When the potential was reversed, a different current was observed. The ratio of these two currents was independent of the resistance of the electrolyte and the surface area of the electrolyte offered to the melt. More importantly, the current ratio was strongly dependent on the activity of the copper and, furthermore, by changing the applied potential, the sensitivity could be increased.