From the procedure developed for computing the In‐Ga‐P ternary phase diagram the pseudobinary phase diagram is generated. The free energies for the ternary liquid and pseudobinary solid are calculated as a function of composition, which establishes the stability of the solid vs. the liquid and shows on what substrate liquid‐phase epitaxial growth is possible. The composition variation of an solid that is deposited during cooling in liquid phase epitaxial growth under various conditions is calculated.