Rectifying CuO/ZnO heterojunctions were successfully fabricated by sputtering thin film p-type CuO onto n-type ZnO polycrystalline substrates. The thermally activated, forward-bias current increased with decreasing oxygen partial pressure pointing to a close correlation between barrier height and adsorbed oxygen at the interface. Both dc I-V and ac impedance measurements were utilized to characterize the deviation from ideality of the characteristics and were found to increase with decreased oxygen partial pressure, (P-O2). An energy band diagram was synthesized and examined in light of the measurements performed in this study.