A model of void formation in Si crystals, from a supersaturated vacancy solution in the presence of nitrogen and oxygen, is developed. At the void formation stage, nitrogen is represented mostly by dimeric species, but the major vacancy traps are nitrogen single interstitials. The trapping is remarkably strong, and yet the voids are easily produced-at a lower temperature and in increased density. These results are in a quantitative agreement with the reported data. A role of the oxygen impurity is to reduce the void surface energy, and also to enhance the pairing/ dissociation rate of nitrogen. (C) 2004 Elsevier B.V. All rights reserved.