We report on x‐ray diffraction measurements of the substitutional carbon contentyin pseudomorphic Si1−yCy(y<0.02) layers grown on (100)‐oriented silicon substrate. The samples were grown by molecular beam epitaxy and investigated during post growth annealinginsituby an x‐ray powder diffractometer. Despite the tensile strain in the 100‐nm‐thick layers and the high carbon supersaturation, the samples were stable up to 800 °C. Beyond this temperature range, the substitutional carbon content decreased exponentially during isothermal annealing. This effect can be explained by the precipitation of the carbon and the nucleation and diffusion limited growth of SiC nanocrystals. Since no strain relief by the introduction of misfit dislocations was detectible, we conclude that contrary to the mechanism of strain relief in Si1−xGex, in comparably strained Si1−yCyepilayers, the main high temperature process is precipitation. ©1995 American Institute of Physics.