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Visible up-conversion in ZnO:Er and ZnO:Er:Yb thin films deposited by RF magnetron sputtering under different O₂-rich atmospheres has been studied. Conventional photoluminescence (325 nm laser source) and up-conversion (980 nm laser source) have been performed in the films before and after an annealing process at 800 °C. The resulting spectra demonstrate that the thermal treatment, either during or post-deposition, activates optically the Er³⁺ ions, being the latter process much more efficient. Moreover, the atmosphere during deposition was also found to be an important parameter, as the deposition under O₂ flow increases the optical activity of Er⁺³ ions. In addition, the inclusion of Yb³⁺ ions into the films has shown an enhancement of the visible up-conversion emission at 660 nm by a factor of 4, which could be associated to either a better energy transfer from the ²F_5/2 Yb level to the ⁴I_11/2 Er one, or …