From van der Waals corrected density functional theory and thermodynamic stability calculations it is found that r-BN is the most stable BN polytype at conditions typically employed for CVD of sp 2 -BN thin films, 1100-1500 °C and 1000-10 000 Pa, suggesting that deposition of h-BN films must be done far from thermal equilibrium. It is also found that the h-BN lattice less liable to expansion than the r-BN lattice and therefore likely to be more sensitive to lattice mismatch to the substrate.