Thermodynamic analysis of the chemical vapour deposition (CVD) of boron nitride was performed for the B-N-H-Cl-He system, with new thermodynamic data for different modifications of boron nitride (hexagonal h-BN, cubic c-BN and wurtzite w-BN). The phase equilibria were calculated for the temperature region 673-2273 K, at total pressures of 1.013 x 10(3) and 1.013 x 10(5) Pa and for a wide range of atomic ratios of elements in the system. The results are presented in the form of CVD phase diagrams which are a good way of illustrating the influence of the system parameters on the formation of stable phases. c-BN is formed in a quasi-equilibrium process at temperatures below 1804 K, the h-BN modification is most stable above this temperature, and w-BN is a metastable phase for all possible variations of the process conditions.