The composition of InxGa1−xAs and InyAl1−yAs epitaxial layers on InP substrates can be measured by high‐resolution x‐ray diffraction (HRXRD) in many cases. If layers are too thick, however, substantial lattice relaxation will occur, requiring multiple asymmetric scans to determine composition. If layers are too thin, they will not produce a distinct Bragg peak. Based upon measurements of both coherent and relaxed layers as well as simulations, we have determined the range of epilayer thickness over which a single HRXRD scan yields the composition of InxGa1−xAs and InyAl1−yAs layers to within 1%. Calibration layers grown within this range allow fast and accurate characterization.