To investigate selective area chemical vapor deposition of Si 1Ϫx Ge x thin films by the alternating cyclic (A.C.) process, a thermodynamic analysis has been performed over extensive temperature, pressure, input gas ratio, and deposited solid composition ranges. In the A.C. approach Si 1Ϫx Ge x thin film deposition via the hydrogen reduction of SiCl 4 and GeCl 4 is followed cyclically by etching of spurious nuclei from mask regions via an embedded disproportionation reaction. The embedded disproportionation reaction between SiCl 4 , GeCl 4 , and the Si 1Ϫx Ge x nuclei is made dominant when the hydrogen flow is interrupted cyclically. The thermodynamic calculations have been carried out via the computer program, SOLGASMIX, which is based on the minimization of the system's Gibbs free energy, and also using a first principles approach as an integrity check. These calculations have indicated that selective area deposition of Si 1Ϫx Ge x thin films by the A.C. method is feasible. The analysis has also defined the parameter space in which to conduct the selective area deposition using the A.C. process.