We use Raman scattering to study phase transition in the graphitic g-BC8 phase and graphite at high pressure up to 84 GPa. The E2g Raman active mode of graphite (G peak) can be detected up to 84 GPa. We demonstrate that there is (1) a phase transition in g-BC8 and in graphite at 35 GPa and (2) that above 35 GPa, the g-BC8 and graphite transform under high pressure to possibly fully sp3-bonded, disordered hp-BC8, and hp-C phases. Below the phase transition, a polynomial fit to the G peak position versus pressure data yielded a quadratic relation; above the phase transition, it demonstrates linear behavior. The phase transition at high pressure in BC8 system and graphite is reversible. Quenched hp-BC8 and hp-C phases have the Raman spectrum typical to that of the graphitic phases. Copyright © 2013 John Wiley & Sons, Ltd.