We present experimental results obtained in H-implanted GaN and He- and Li-implanted ZnO. The ion energies were varied in the range 100-850 keV, and the implantation fluences in the range 5 x 10(13) to 1 x 10(18) cm(2). In addition, conventional and. ash anneals at temperatures 500-1400 degrees C were performed on the ZnO samples. The data obtained with a slow positron beam show that vacancy clusters are formed in as-implanted samples with fluences above 1 x 10(17) cm(2). Below this value only single vacancies are detected after implantation, but vacancy clusters can be formed and subsequently dissociated by thermal annealings. (C) 2008 Elsevier B.V. All rights reserved.