Thermodynamic information is used to correlate the composition of germanium-gallium solid solutions with the temperature, pressure and composition of a gas mixture with which the solid is in equilibrium. At temperatures below 1000°K, and gallium concentrations in excess of 1016 atoms cm3 of solid,the dominant gas phase species are GaI, GeI2 and GaI3 with the mole fraction of GeI2 in the gas mixture becoming very small at gallium concentrations greater than 1018 atoms cm-3 and temperatures below 900°K. Epitaxial gallium doped germanium layers were grown and their composition found to be close to the calculated equilibrium composition. The growth rate agreed quantitatively with that predicted on the assumption that gaseous diffusion was the rate limiting process. The very low mole fraction of GeI2 prevents rapid quasi equilibrium single crystal growth of heavily doped material at low temperatures. © 1965.