In the present work, the melting and phase transitions of three phosphorus allotropesorthorhombic, rhombohedral and simple cubic (corresponding crystal structures are presented in Fig. ) -have been studied up to 10 GPa, and the data obtained were used for thermodynamic calculations of the phase relations in phosphorus. The thermodynamic analysis based on our experimental and literature data allowed us to construct equilibrium p-T phase diagram of phosphorus at pressures up to 12 GPa.