Abstract: The paper reports the results of thermodynamic analysis and experimental studies on the reactions in the source zones of the Ga-HCl-NH₃-carrier gas system for depositing GaNthin fimls.It ispredicted theoretically that at higher temperatures and in an usual partial pressure range of input HCl hydrogen chloride transforms completely into chlorides among which GaCl is the dominant species transporting toward deposition zone.Experimental study showed that the reactions in Ga source zone did not reach equilibrium and a significant influence of gas flow condition and temperature on the transformation of HCl was found.In the deposition temperature range (1173-1323K) and NH₃ initial partial pressure range (0.2 - 0.4atm) the thermal decomposition of NH₃ was not considerable that NH₃ would essentially be the species containing Nand reacting with GaCl in deposition zone.