This study investigates the stability of Metastable Si1-yCy/Si heterostructures during rapid thermal annealing (RTA) over a temperature range of 1000 – 1150° C Heterostructures of Si1-yCy/Si and Si1-x-yGexCy/Si (x=0.077, y ≤ .0014) were formed by solid phase epitaxy from C implanted, preamorphized substrates using a 30 Minute 700° C anneal in N2. The occupancy of C in substitution lattice sites was monitored by Fourier Transform Infrared Absorption spectroscopy. The layer strain was monitored by rocking curve x-ray diffraction and the structural changes in the layers were determined using plan-view and X-sectional transmission electron Microscopy (TEM). For anneals of 1150° C or above, all the substitutional C was lost from the Si lattice after 30 seconds. TEM verified that the strain relaxation was the result of C precipitating into highly aligned βSiC particles rather than by the formation of extended defects. No nucleation barrier was observed for the loss of substitutional C Preliminary results will also be discussed for Si1-x-yGexCy/Si heterostructures where there is the additional factor of the competition between strain energy and the chemical driving forces.