A solid electrolyte oxygen sensing cell employing yttria-stabilized zirconia (YSZ) was used to determine the oxygen content and activity in molten copper on a laboratory scale at temperatures between 1200 and 1400°C. The effects of temperature and time of treatment were investigated on the deoxidizing process with Cu3P. The amount of Cu3P added ranged from 1 to 3% of the mass of molten copper. At 1200 and 1300°C, fair agreement was obtained between the oxygen contents calculated from the measured probe EMF and those obtained by chemical analysis. It was found that the oxygen is more efficiently removed at lower temperatures. A metallographic analysis of rapidly cooled samples showed that oxygen and phosphorus are present in the copper mainly as oxide dendrites.