The variation of bandgap with composition for gallium—indium phosphide alloys is discussed. Γ and X band energies are calculated using a parabolic interpolation. At room temperature, all existing experimental data are accounted for by assigning the cathodoluminescence line to a donor—valence band transition (as shown by low temperature photoluminescence). The crossover composition and energy are: x c = 0.66 ± 0.02, E g = 2.210 ± 0.005 eV at 300° K.