In this study we discuss the microstructural changes after electron and proton irradiation and the thermal evolution of the radiation induced defects during isochronal annealing of single crystals irradiated either with 3 MeV protons or with 1 or 2 MeV electrons, respectively. The investigations were performed with positron lifetime and Doppler-broadening measurements. The differently grown ZnO single crystals show positron bulk lifetimes in the range of 159-173 ps.