We have investigated the properties of homogeneous spinel Co3-yZnyO4 (y=0, 0.25, and 1) and wurtzite CoxZn1-xO (0≪x≪0.1) films. Air annealed Co3-yZnyO4 films were found to develop only a small magnetic moment (∼10-3μB/Co). Conversely, these samples exhibited greatly enhanced magnetic moments after vacuum annealing. The vacuum annealed films reveal the presence of Zn doped CoO, but no detectable Co metal clusters. When field cooled to 10 K, the hysteresis curves of the vacuum annealed samples are displaced along the magnetization axis, which we attribute to uncompensated surface spins.