Hydrogen (H) at a zinc vacancy (V Zn ) in ZnO is identified by electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM). In ZnO irradiated by 2 MeV electrons, a doublet EPR spectrum, labelled S1, was observed. The doublet structure and the accompanied weak satellites are shown to be the allowed and forbidden lines of the hyperfine structure due to the dipolar interaction between an electron spin S=1/2 and a nuclear spin I=1/2 of 1 H located at a V Zn . The involvement of a single H atom in the S1 defect is further confirmed by the observation of the nuclear Zeeman frequency of 1 H in ESEEM experiments. We show that at a V Zn , H prefers making a short O-H bond with one O neighbour and is off the substitutional site, forming a low symmetry C 1 defect. In this partly H passivated V Zn , the unpaired electron localizes on the p orbital of another O neighbour of V Zn , and not on H.