A study on thermodynamics of the various steps involved in the polysilicon manufacture in the Siemens process is presented in this paper. The synthesis of SiHCl3 (TCS), silicon deposition from SiHCl3 and hydrogenation of by-production SiCl4 (STC) for recycle are the main processes studied in this paper. Based on the thermodynamic data for related pure substances, the relation of (n(Cl)/ n(H))(Eq) (i.e. the mole ratio of Si to Cl at equilibrium) and the feeding mole ratio of Cl to H (n(Cl)/n(H)) was plotted in the Si-Cl-H system. Then, the effects of temperature, pressure and feed fraction on TCS selectivity in the synthesis of SiHCl3 process, polysilicon yield in the silicon deposition process and SiCl4 conversion ratio in the SiCl4 hydrogenation process have been studied. Useful conclusions for improving these processes have been drawn. Finally, the optimum conditions have been obtained and compared with limited published data in the literature. The results show that comparison with the limited reported values in the literature indicates that the operating conditions are close to those predicted by the thermodynamics for all three reactions. However, the reported conversion or yield is lower. It is hoped that this paper makes a contribution as future polysilicon plants move to higher capacity and seek a reduction in the cost of production.