The thermal stability of molecular beam epitaxy grown Si1-x-yGexCy/Si heterostructures (0≤x≪0.30, y∼0.008) was studied using infrared absorption spectroscopy. The local vibrational mode of C in Si and Si1-x-yGex was used to quantify the loss of C atoms from substitutional sites with high temperature annealing. The activation energy (Ea=4.9 eV) for the loss of substitutional C achieved a maximum for the strain compensated alloy (x∼0.1). An additional increase of Ge content resulted in a rapid decrease in Ea, which was found to be 3.4 eV for x∼0.27. The nonmonotonic behavior of Ea on Ge content is explained by the effect of the interface strain between the epitaxial layer and Si substrate. © 1998 American Institute of Physics.