New experimental results for cubic boron nitride (cBN) formation in the system Mg 3 N 2 -BN are presented and discussed. Generally, two different regions may be distinguished. At high pressures and temperatures precipitation processes from a melt via nucleation and growth mechanisms dominate. Owing to the occurrence of two different intermediate compounds, Mg 3 BN 3 at the beginning of the cBN-formation region and a newly discovered compound more BN-rich at higher temperatures, the kinetics changes remarkably in that region. A generally different formation mechanism is observed at low pressures (0.5 GPa) and only at enhanced temperatures (480 K) after a quench. Under these conditions the new BN-rich compound becomes unstable and decomposes into cBN and another product by fast solid-solid transformation processes. consequences for the resulting phase diagram are discussed.