The structural and morphological characteristics of flat BN coatings processed by chemical vapour deposition, using BCl3−NH3−H2 gas mixtures at low pressure (P < 1 kPa), have been investigated as a function of the deposition temperature (ranging from 900 °C to 1400 °C) and the total gas flow rate. The resulting BN coatings are mainly turbostratic but with heterogeneous microstructures, i.e. mixtures of poorly and highly organized domains. The structural homogeneity and the degree of crystallization depend notably on the nature of the dilution gas (either H2 or Ar) and the depletion of gas species. The decrease of the apparent activation energy from 80 kJ.mol-1 below 1200 °C to 40 kJ.mol-1 above 1200 °C reflects a change in the deposition regime. Ex situ Fourier transform infrared analysis of the residual gas mixture allowed intermediate species leading to poorly or highly organized BN to be tracked and then connected with the main reaction paths leading to the different BN coatings.