X‐ray diffraction and electron probe microanalysis were used to determine the phase equilibria in the In‐Pt‐P system at 725 and 600 °C. The observed binary phases were consistent with the assessed phase diagrams from the literature. Three ternary phases were found. The major difference between the 725 and 600 °C isotherms was the phases in equilibrium with InP. At 600 °C, the binary phases Pt3In7and PtP2were in equilibrium with InP. However, a tie‐line transition from InP‐Pt3In7to In‐PtP2was found between 600 and 725 °C. The binary phases which were found to be stable in thin film form on InP were consistent with the phase equilibria data. From the tie‐line change, the enthalpy of formation of PtP2at 298 K was estimated to be −350±40 kJ/mol.