Refractive indices of InGaAs/InAlAs multiquantum‐well layers grown by molecular‐beam epitaxy are determined by reflectance measurements as a function of photon energy and barrier width, i.e., average Al content. These results are well explained by the single‐oscillator model; oscillator energy (E0) and dispersion energy (Ed) values vary linearly with average Al content. These values are, however, found to differ from the values obtained by the linear approximation of the parameters for binary alloys. Discussion of this discrepancy is also presented.