It has been reported that boron segregates to single-crystal Si1-xGex layers from silicon during thermal anneals. In this work, we find that boron segregates even more strongly into single-crystal Si1-x-yGexCy, as has been previously reported for polycrystalline films. This effect is also observed in single-crystal Si1-yCy. Segregation coefficients range from 1.7 to 2.9 for annealing temperatures in the 800-850° C range. In a Si1- yCy layer with 0.4% carbon, most of the segregation is reversible if the carbon is removed by an oxidation-enhanced out-diffusion process. This argues against the formation of immobile B-C defects as the driving force for the segregation. Gradients of interstitial silicon atoms, created by high concentrations of substitutional carbon, are presented as a driving force capable of causing the segregation seen in the experiments. © 2005 The Electrochemical Society. All rights reserved.