Effect of heat treatment on carbon in cast multicrystalline silicon (mc-Si) has been studied by means of Fourier Transmission Infrared Spectroscopy. Carbon is found to be involved in the formation of as-grown precipitates in me-Si with higher oxygen content. The experimental results reveal that carbon is difficult to precipitate in me-Si with lower oxygen or higher nitrogen concentration during annealing in the temperature range from 450degreesC to 1150degreesC. Carbon can enhance the nucleation of oxygen precipitates at lower temperature (<850degreesC). Although carbon does not affect the amount of oxygen precipitates at higher temperature (>950degreesC), it is suggested that carbon diffuses into oxygen precipitates by the enhancement of silicon self-interstitials. The experiments point out that preannealing at 750degreesC enhances the decrease of substitute carbon concentration during subsequent annealing at 1050degreesC. Dislocations and grain boundaries in mc-Si do not affect carbon thermal treatment properties, (C) 2002 Elsevier Science B.V. All rights reserved.