For an ideal Si-Ge solid solution deposited from an ideal vapor phase in a two phase solid solution-vapor system, the equilibrium constant, K-Si(SS), for the global process has been derived in terms of the equilibrium constant fdr the deposition of a pure solid phase, K-Si(S), and the solid solution mole fraction of silicon species, N-Si(SS), and has been shown to be K-Si(SS) = K-Si(S)/N-Si(SS). Derivation of the relationship for the germanium species is symmetrical to that for silicon and given by K-Ge(SS) = K-Ge(S)/N-Ge(SS) = K-Ge(S)/(1 - N-Si(SS)).