Based on first-principle total-energy calculations, we show that the majority of vacancies are trapped by interstitial oxygen in silicon wafers with a typical oxygen concentration of about 1018 cm-3. Vacancies and interstitial oxygen form so called A centers with a binding energy of 1.7±0.4 eV. As a consequence, the density of bound vacancies exceeds the equilibrium density of free vacancies by several orders of magnitude and effective vacancy diffusion coefficients in Si are reduced in comparison to the diffusion coefficient of free vacancies. However, we find that trapping of vacancies alone cannot account for the large discrepancies between previously reported diffusion coefficients for vacancies. © 2001 American Institute of Physics.