Carbon doping strongly suppresses the diffusion of boron. This is very important for later high temperature processing steps, e.g. for the fabrication of a high doped SiGe-base in a heterojunction bipolar transistor. We investigated (with a novel method) the segregation of C in Si (100) at growth temperatures between 500 and 700 degreesC. A C-doped Si layer was grown by MBE under constant C and Si fluxes. The growth temperature was switched between 300 degreesC and the investigated growth temperature. As the temperature was switched, the surface-accumulated C concentration was perturbed, causing a spike or depression in the SIMS depth profile. The surface segregation at a given temperature was measured by the amount of adlayer density in the spike. Clear evidence of the surface segregation of carbon in Si is found by these experiments.