The perfection of dislocation‐free silicon crystals has been followed as a function of heat treatment at 1000°C by x‐ray anomalous transmission. For floating‐zone silicon crystals carefully grown to exclude oxygen and carbon, the intensity of the x‐ray anomalous transmission does not change following heating for several hundreds of hours at 1000°C. This contrasts with previous measurements on floating‐zone silicon where the intensity decreased drastically following the same heat treatment. Suitable experiments have ruled out carbon or vacancies as being responsible for the effects observed in the floating‐zone crystals. A Czochralski crystal with 7.5 × 1017cm−3of oxygen shows a rapid decrease in perfection when heated at 1000°C. The x‐ray anomalous transmission is more sensitive to the heat treatment effects in the early stages of precipitation than the usual infrared 9‐&mgr;m absorption measurements. The defects contributing to the decreased x‐ray anomalous transmission have been detected by x‐ray section topographs. Although the precipitate has not been directly identified, these and previous experiments allow the conclusion that oxygen clustering is responsible for the heat treatment effects observed. The results are discussed in the light of recent theory on the effect of clustering on x‐ray anomalous transmission.