Ultrapure ZnO nanopowders were synthesized via vapor-phase based methods under oxygen deficient conditions. The type and relative proportions of intrinsic point defects were studied by photoluminescence (PL) and EPR spectroscopies performed under strictly controlled conditions. Besides coupled PL/EPR signals recently assigned to Zni + (2.80 eV / g=1.96) two green emissions were systematically detected at 2.50 eV and 2.22 eV without EPR counterparts whereas their contributions were observed to depend on synthesis' oxygen partial pressure (PO2). Among diamagnetic defects likely to be formed in O2-poor conditions, Zni 0 and Zni 2+ were discarded based on their reported energy levels-with transitions associated rather to match the violet light. Conversely, the involvement of oxygen vacancies (VO 0 and VO 2+) as recombination centers for the green emission in ZnO was supported by Raman and XPS data. In line with the expected trends based on formation energies, the always dominant green luminescence (2.50 eV) was assigned to VO 2+ and the weaker one (2.22 eV) to VO 0. The involvement of an electron containing defect (VO 0) was confirmed by visible light absorption observed in DR UV-Vis spectra. We also showed that the Zni + / VO 2+ ratio can be tuned by PO2 or by the choice of static or flow synthesis conditions. Overall, this study demonstrates that by controlling the conditions during synthesis, processing and spectroscopic investigations, the 2 ultrapure ZnO nanopowders represent reliable models for the identification of photoluminescent crystal defects-an approach that can be widely applied on other systems.