In this study, we investigated the dominating intrinsic defect levels in CuInSe₂ by means of temperature- and excitation-dependent steady-state photoluminescence (PL). The polycrystalline thin films were prepared by rapid thermal treatment of metallic alloys in H₂Se-Ar. Attention was mainly focused on the influence of post-growth treatments (in Ar-H₂ and O₂) on the defect structure of these films. The PL spectra of slightly In-rich as-grown films were dominated by a donor-acceptor pair transition at 0.92 eV. The defect levels are ascribed to V_Cu and V_Se and their activation energies have been determined to be 45 and 95 meV, respectively. Post-growth treatment in Ar-H₂ resulted in a shift of these peaks to higher energies and a significant increase in the intensity. In contrast, annealing in O₂ resulted in a shift of the peak to lower energies and to a dramatic reduction in intensity, compared with the as-grown samples. These effects were also found to be reversible. The change of the PL spectra on oxygen and hydrogen annealing is explained by the change of the density of V_Se defects. This is explained by a model in which oxygen can occupy a V_Se site owing to coordinatively unsaturated In at the grain boundaries. The V_Se concentration and therefore the device quality of these CuInSe₂ thin films can thus be controlled either by a post-growth annealing step or during the growth process itself.