Internal strain energies for quaternary III‐V compound alloys of the type AxB1−xCyD1−yhave been estimated. The strain is attributable to the difference in bond length among the constituent compounds. Results are shown in the form of contour of the energy. The energies in AlGaAsP are relatively small; the maximum value is about 4 kJ/mol, and those in AlInPSb are large; the maximum is about 50 kJ/mol. The alloys are concluded to be hardly miscible by ordinary growth methods in the domain of strain energies larger than 19 kJ/mol. The qualities of the alloys can be predicted by the internal strain‐energy charts presented in this study. The effect of movement of atoms from the virtual crystal‐lattice sites is discussed. Comparison to the enthalpy of mixing is also made.