As‐grown Czochralski silicon samples with different oxygen concentrations have been heated at temperatures in the range 350–500 °C. Oxygen loss during anneals at low temperatures (T≤400 °C) is shown to follow second‐order kinetics and measurements led to values of oxygen diffusivity that were larger than normal by a factor of ∼3, assuming the capture radius for dimer formation was 5 A˚. Variations in the rate of [Oi] loss during more extended anneals could be explained if oxygen diffusion was initially enhanced but tended to its normal value as the anneals progressed. Much greater initial enhancements were derived from similar measurements for samples which had been hydrogenated by a heat treatment in H2gas at 1300 °C for 30 min followed by a rapid quench to room temperature, and the enhancements were consistent with values derived from measurements of the relaxation of stress‐induced dichroism. At higher temperatures (T≥450 °C) the measured rates of [Oi] loss were less than the expected rate of Oi‐Oiinteraction and tended to vary with increasingly high powers of [Oi]. Modeling of the clustering process demonstrated that the reductions could be explained if the oxygen dimers were present in a quasiequilibrium concentration throughout the anneals. The establishment of this equilibrium appears to require that oxygen dimers diffuse much more rapidly than isolated Oiatoms. The kinetics of oxygen loss over the whole range of temperatures can then be explained if dimer clustering leads mainly to increases in concentrations of agglomorates containing large numbers (≥8) of oxygen atoms. It is therefore possible to account for thermal donor (TD) formation based on the formation of different sizes of oxygen clusters, although the possibility that self‐interstitials are involved in TD formation is not excluded. ©1995 American Institute of Physics.