Defects in the separation by implanted oxygen (SIMOX) wafers were studied by monoenergetic positron beams. For the as-implanted specimen, vacancies introduced by ion implantation were found to form vacancy-oxygen complexes. After high temperature annealing, a drastic increase in the formation probability of positronium was observed. This suggests an introduction of open-spaces by the formation of an amorphous SiO2. The present investigation shows that positrons provide a nondestructive probe for the detection of defects in SIMOX wafers and microstructures of SiO2. © 1993 J.C. Baltzer AG, Science Publishers.