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 effective I sinks in the Si matrix at these temperatures. (iv) An enhancement of C diffusivity in the presence of excess I plays an important role in increasing the precipitate growth rate, particularly at low temperatures for which no efficient I sinks are available in the Si matrix. © 1995 American Institute of Physics.