High temperature (1200°C) HCI oxidation treatment has been employed to float-zone (FZ) silicon wafers (625μm thick) containing swirl defects in order to study their diffusion characteristics. In treated wafers, swirl defects can be eliminated from both surfaces up to a depth of ∼30μm. In the bulk of the wafers, however, large swirl defects (A-swirls) rearrange themselves into many small defects. The untreated portions of wafers contain large swirl defects (A-swirls) that extend up to both surfaces. Since swirl defects are primarily clusters of silicon self-interstitials, their rearrangement in the bulk and elimination from the surfaces demonstrate that migration of interstitials takes place on a large scale and is not confined to SiO2/ silicon interface only. The above observations appear to provide direct evidence for the dominant role of self interstitials for diffusion mechanism in silicon at high temperature and can be rationalized in terms of an interstitialcy mechanism. Alternatively, however, dominance of interstitials can be related to a higher migration energy of vacancies proposed in a model where both species coexist at high temperature. The preference of one model over another must await theoretical calculations of diffusion energetics derived from both models.