To investigate selective area chemical vapor deposition of Si1-xGex thin films by the alternating cyclic (A.C.) process, a thermodynamic analysis has been performed over extensive temperature, pressure, input,aas ratio, and deposited solid composition ranges. In the A.C. approach Si1-xGex thin film deposition via the hydrogen reduction of SiCl4 and GeCl4 is followed cyclically by etching of spurious nuclei from mask regions via an embedded disproportionation reaction. The embedded disproportionation reaction between SiCl4, GeCl4, and the Si1-xGex 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 Si1-xGex 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.