The formation and evolution of the prominent and so-called E3 center in ZnO has been studied by in-situ deep level transient spectroscopy measurements after on-line implantation of hydrogen (H) and deuterium (D) ions at sample temperatures of and . The formation of E3 is shown to involve migration and subsequent trapping of interstitial hydrogen (Hi), or deuterium, and starts to occur already below 200 K. The concentration of implantation-induced E3 centers is rather unstable and decreases gradually at temperatures around 300 K by an annealing process obeying first-order kinetics. The process exhibits an activation energy of and involves presumably trapping of migrating Hi's leading to passivation of the E3 centers. A kinetics model is presented showing good agreement with the experimental data and where the E3 center is assumed to be a complex between a Zn vacancy and three hydrogen atoms . Further, the concentration of E3 centers is found to decrease rapidly during annealing in forming gas ambient at room temperature and then to recover gradually during subsequent annealing in vacuum, indicating a defect ‘core’ of the E3 center able to accommodate more than one H atom.