The chemical identification of donor bound excitons in ZnO has been studied using radioactive ions. Implantations of 117Ag – which decays to radioactive Cd and In – have enabled the identification of the I2 optical feature as being the ionized donor counterpart of I9, one of the most prominent optical features in the photoluminescence spectrum of ZnO. Both of these lines are consistent with In occupying a Zn site.