Positron Lifetime spectroscopy and cathodoluminescence were employed to study luminescence centers in ZnO. The samples were high-purity polycrystalline ceramics sintered at temperatures ranging from 800 to 1400 degrees C for 2 to 40 h. Scanning electron microscopy shows that as annealing temperatures and/or times increase, the average grain size increases and can reach 30 mu m for samples sintered at 1200 degrees C. At the same time, the positron bulk Lifetime preaches theoretically estimated single-crystal values, while the integrated luminescence intensity increases significantly. A further increase of the sintering temperature beyond 1200 degrees C results in a decrease in the luminescence intensity, in good agreement with the only weak luminescence observed in single-crystalline material. The positron lifetime spectra clearly show the existence of one dominant vacancy-type defect, most likely a complex involving V-Zn, or the divacancy, V-Zn V-O, independent of sample thermal history. The concentration of this center steadily decreases with increasing sintering temperature. It is concluded that the yellow luminescence centers are related to charged zinc vacancies trapped in the grain boundary regions. We propose that the observed broadness of the spectra Likely originates from the modification of the electronic configuration of the luminescence centers due to their complex environment. A direct connection between the positron and the luminescence results could not be established; instead, they appear to reflect two relatively independent aspects of the samples. It could be shown, however, that positron annihilation measurements can be used effectively to monitor the evolution of the microstructure of the samples, in good agreement with scanning electron micrographs.