Oxygen (O) and carbon (C) coprecipitation in Czochralski Si is studied in terms of a diffusion‐limited growth model. The interfacial energy increase upon C incorporation into oxide precipitates as well as the changes of O and C concentrations in the Si matrix with annealing time have been taken into account. A comparison of the model predictions with available experimental data has led to the following conclusions: (i) Regardless of the C content in the crystal, it is necessary to introduce sinks for the precipitation‐induced excess Si self‐interstitials (I) in the matrix for high annealing temperatures. (ii) At annealing temperatures below about 1000 °C, the enhancement effect of C on O precipitation results primarily from an increase in the precipitate density. (iii) The transition in the C precipitation behavior observed in C‐rich Si crystals at annealing temperatures around 800 °C is related to a change in the availability of effectiveIsinks in the Si matrix at these temperatures. (iv) An enhancement of C diffusivity in the presence of excessIplays an important role in increasing the precipitate growth rate, particularly at low temperatures for which no efficientIsinks are available in the Si matrix. ©1995 American Institute of Physics.