Positron lifetime and coincidence Doppler broadening (CDB) techniques, combined with first-principles calculations of positron annihilation characteristics, are employed to study vacancy-oxygen (VO) complexes in Czochralski-grown silicon (Cz-Si). In the experiments, the positron lifetimes and CDB spectra were measured as functions of post-irradiation annealing temperature for a series of electron-irradiated Cz-Si samples. Though the longer lifetimes for the defects are nearly constant at about 300 ps, the CDB spectra exhibit a distinct stage around 350 degreesC, indicating a marked change in the defect nature after the post-irradiation annealing. These experimental results are compared with the calculated positron lifetimes and CDB spectra for various vacancy-oxygen complexes in Si. This comparison clarifies that, after annealing at 350 degreesC, the irradiation-induced divacancies and A centers aggregate into more stable V3O and V4O2. (C) 2002 Published by Elsevier Science B.V.