High quality, low defect density, single crystalline silicon/germanium alloys have been grown on Si(100) substrate wafers in a low temperature UHV-CVD reactor. Using a silane/germane gaseous source, the growth rate of the epitaxial layer increases from 4 angstroms/minute with no germane present to 82 angstroms/minute with 12.7% germane present in the reaction gas mixture at 550C. The germanium/silicon ratio in the deposited alloy is a factor of two greater than the germane/silane ratio in the reaction gas mixture. The kinetics of this effect are studied and correlation to UHV hydrogen thermal desorption from single crystal silicon-germanium alloys are made.