Direct evidence for the co-aggregation of oxygen and carbon is shown by secondary ion mass spectroscopy for a carbon-doped Czochralski silicon crystal subjected to a heat treatment at 1000 °C. In conjunction with infared absorption data, the carbon enhancement effect on oxygen precipitation at 1000 °C in high carbon silicon is explained with the catalytic effect of carbon in modifying the interfacial energy or the point defect ambient at the oxygen precipitate surface.