Oxygen-related defects in Separation-by-IMplanted OXygen (SIMOX) wafers and their annealing behavior are studied by variable-energy positron annihilation spectroscopy. After implantation of 180 keV oxygen into Si substrates at a dose of 4 x 10(17) cm(-2), two kinds of vacancy-type defects are observed by positrons, i.e. vacancy clusters and vacancy-oxygen complexes. Their sizes are larger than and close to V-6, respectively. According to the annealing behavior of the vacancy cluster, there are three annealing stages. A detailed discussion of the annealing behavior of the two defects is presented. (C) 2002 Elsevier Science B.V. All rights reserved.