Heat contents of the In-P and Ga-P systems were measured by a drop-calorimeter in the respective concentration range of N_p=0.05 to 0.50 (InP) and N_p=0.02 to 0.10, at the temperature ranges of 800 to 1450 K and 800 to 1550 K. A thermodynamic analysis method was applied to calculate thermodynamic quantities such as the activities of related components in the liquid In-P and Ga-P systems and the free energies of mixing in the liquid In-P and Ga-P systems. The activity of In in the liquid In-P system at 1473 K showed a negative deviation and a slight positive deviation from Raoult’s law in the compound side and in the In-rich region, respectively, while the activity of P showed a large negative deviation. The activity of Ga in the liquid Ga-P system at 1573 K showed a slight positive deviation from Raoult’s law, and that of P showed a large negative deviation. Based on the obtained thermodynamic data, total pressure- or partial pressure-temperature-composition correlation diagrams were constructed for the In-P and Ga-P systems. Total pressure of phosphorus gas species and partial pressure of P₂ showed a drastic increase with increasing temperature and increasing phosphorus concentration.