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ZnO quantum dots with relatively narrow size distribution were produced by the presented precipitation method. Theoretical results from the literature were correlated to Zn-rich and O-rich preparations of ZnO. The investigation of the defect evolution was performed by photoluminescence (PL) measurements and electron paramagnetic resonance (EPR). By a deconvolution of PL spectra, the often mentioned defect-related green luminescence with line centers between 2.72 and 2.26 eV was observed. Since all defect centers have different defect formation enthalpies, additional information on the formation of these centers via annealing was collected. This data led us to the conclusion that VZnshows more prominent properties than usually reported. The VOdefect is assigned to be responsible for the blue-light emission. The investigation of nonstoichiometric ZnO including its annealing revealed that the EPR signal at g= 1.959 can be assigned to core defects with a weak coupling to remote hydrogens from water molecules. This assignment is corroborated by the detection of hydrogen-related resonances by electron double resonance (ENDOR) spectroscopy.