Phase relations in the B–BN system have been studied ex situ and in situ at pressures 2-20 GPa and temperatures up to 2800 K. The evolution of topology of the B–BN phase diagram has been investigated up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on phase equilibria at high pressures and high temperatures. There are two thermodynamically stable boron subnitrides in the system i.e. B13N2 and B50N2. Above 16.5 GPa the B50N2 ⇄ L + B13N2 peritectic reaction transforms to the solid-phase reaction of B50N2 decomposition into tetragonal boron (t'-B52) and B13N2, while the incongruent type of B13N2 melting changes to the congruent type only above 23.5 GPa. The constructed phase diagram provides fundamentals for directed high-pressure synthesis of superhard phases in the B-BN system.