The process of reactive sputtering is influenced by the reactive gas pressure and the rate of sputtering because the glow-discharge characteristics vary considerably in the presence of reactive gas. The discharge characteristics during magnetron sputtering of copper in an argon and oxygen atmosphere have been investigated in the present study. The variation in the cathode potentials has been explained in terms of negative ion formation and target poisoning effects. It has been found that the rate of deposition and the oxygen partial pressure influence target poisoning, which in turn influences the discharge characteristics and rate of deposition. The properties of films deposited under different conditions have been correlated with the discharge characteristics and target poisoning.