The refractive indices of In0.49Ga0.51P, In0.49Al0.51P, and In0.49Ga0.29Al0.22P, lattice matched to GaAs grown by molecular‐beam epitaxy, are determined from double‐beam reflectance measurements for photon energies ranging from 0.6 to 1.3 eV. Variation of the In0.49Ga0.51−xAlxP, refractive index with Al compositionxand photon energy is calculated according to the single‐effective‐oscillator model. These analytical results are then compared with experimental data.