Melting and solid-state phase transitions of phosphorus allotropes have been in situ studied in the 2-11 GPa pressure range using electrical resistivity measurements and synchrotron X-ray diffraction. While melting curves of rhombohedral and simple cubic allotropes exhibit positive slopes 58 ( 2) and 300 ( 90) K/GPa, respectively, the rhombohedral-simple cubic phase boundary has negative slope of -420(70) K/GPa), and the rhombohedral-polymeric liquid -simple cubic triple point is located at about 8.3 GPa and 1490 K. Calculations of phosphorus phase equilibria have been performed using our experimental and literature data in the framework of the