Cubic BN (c-BN) with sp -hybridization and diamond structure is the most wellknown among BN phases. In 1957, Wentorf synthesized c-BN from a mixture of boron and nitrogen at high temperature and pressures. The discovery of "Borazon" catalyzed work on the other polytypes of BN, foremost sp 2 -hybridized hexagonal BN (h-BN) "white graphite" and to some extent the less studied wurtzite form (w-BN) that is similar to "hexagonal diamond". In addition to h-BN a rhombohedral sp -hybridized BN phase (r-BN) was reported 1958 by Hérold. The two sp -hybridized BN polytypes differ in their stacking sequence of the basal planes along the c-axis; h-BN exhibits an ABAB... stacking while r-BN has an ABCABC... stacking sequence. Furthermore, the basal planes in h-BN are rotated 180°, with respect to the previous basal plane, around the caxis ([0001]), while in r-BN, each next basal plane is instead shifted along the direction by 1.45 Å. The very close structural similarity between h-BN and r-BN is demonstrated in identical in-plane lattice parameters, 2.504 Å, and spacing between the basal planes, 3.333 Å. As a thin film, BN is a promising material for many electronic applications, where the polytype, h-BN or r-BN, are likely to affect the electronic properties: h-BN is reported to have a slightly larger band gap than r-BN; 5.955 9 and 5.7 10 eV, respectively. Typically, sp 2 -BN films have been deposited by thermally activated Chemical Vapor Deposition (CVD), i.e. close to thermal equilibrium conditions. Epitaxial growth of sp 2 -BN thin films by CVD is typically carried out from triethylboron, B(C2H5)3 (TEB), and ammonia, NH3 at low pressures of 1000 -10000 Pa and temperatures in the range of 1100-1500 °C (1400-1800 K). The high temperatures needed limit the number of available substrate materials, thus, growth of epitaxial films has been restricted to α-Al2O3 (0001) and 4H/6H-SiC (0001) substrates, yielding films oriented around the caxis. 12-15 sp 2 -BN as a two-dimensional material will consist of a single basal plane with hexagonal symmetry and it is therefore, correctly, refer to as h-BN. However, the authors have previously pointed out the difficulty in structural determination of sp 2 -BN films grown along the c-axis, i.e. the [0001] direction, of thicknesses exceeding a few nanometers, and advised to use a more extensive characterization approach to fully understand if a film of sp 2 -BN is h-BN or r-BN. In this letter, the authors use van der Waals corrected density functional theory and thermodynamic stability calculations to study the stability of the sp 2 -BN polytypes at temperatures and pressures relevant to CVD. From the results it is found that at typical CVD temperatures and pressures, r-BN is the phase predicted to be most stable by thermodynamics.