Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6MeV H + ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378eV (10K). The irradiation creates an intense and narrow emission at 3.368eV (10K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368eV emission indicate its origin as a hydrogen at oxygen vacancy type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164±1ps) and irradiated crystal (175±1ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be 4×10 17cm 3 (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of 175ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed. © 2012 IOP Publishing Ltd.