A mass spectrometer has been coupled to an open‐tube, vapor‐phase, crystal growth apparatus in order to study vapor‐phase processes leading to the deposition of crystals. Studies of the and Cl2 reactions with gallium showed that is the only gallium chloride formed. Studies of decomposition showed that very little decomposes under given experimental conditions, but this can be changed by the presence of W and Pt. Thermochemical data were collected or calculated for all species detected in the system. The reaction mechanisms leading to the deposition of are discussed. It is concluded that the reaction controlling the deposition of can be written as Comparison of the experimentally determined vapor composition with one predicted from the available thermochemical data showed that important deviations from the thermochemical equilibrium occur in our system.