In this work, the influence of UV irradiation (254 nm) on the precipitation of ZnO nanoparticles was studied. UV irradiation was found to induce photocorrosion of originating ZnO nanoparticles, which resulted in the formation of oxygen and zinc vacancies confirmed by photoluminescence and positron annihilation spectroscopy. The oxygen vacancies were formed by the reactions of hydroxyl radicals with oxygen atoms of the ZnO lattice. Positron annihilation spectroscopy also revealed the presence of oxygen and zinc divacancies and the clusters of 5 zinc vacancies and 10 oxygen vacancies, which enhanced the orange-red photoluminescence of ZnO nanoparticles. Using electron transmission microscopy and high resolution electron transmission microscopy a median of the ZnO nanoparticle sizes was estimated at about 17 nm while a median of the ZnO nanocrystal sizes was about 7 nm, respectively. No effects of the photocorrosion on the nanoparticle and nanocrystal sizes were observed. A new mechanism of the Zn(OH)₂ formation based on the reactions of hydroxyl radicals with zinc precursors was suggested.