FTIR exhaust gas analysis is used to establish chemical reactions paths in chemical vapor growth of GaN single crystals. The growth process utilizes the classical Degussa process on a lab scale to synthesize HCN as a transport agent for Ga. With GaCN as Ga and C precursor, the process is very similar to halide vapor phase epitaxy (HVPE), where GaCl is the Ga precursor and in both methods, the source of reactive nitrogen is NH3. Spectra are presented for the pure HCN synthesis and its combination with vapor growth, allowing the effective optimization of growth parameters, especially of gas flows. (C) 2016 Published by Elsevier B.V.