The validity of optical absorption (OA) as a technique for the measurement of strain e11, alloy composition x, and relaxation in InxGa1−xAs epilayers on InP has been examined by comparison with similar measurements by double‐crystal x‐ray diffraction (DCXD). Provided that the strain arising from differences in the thermal contraction of the substrate and epilayer are taken into account, measurements of strain by OA show good agreement with DCXD results, with a dispersion of Δe11=±0.27×10−3. Comparison of alloy compositions given by the two techniques shows similarly good agreement, with a dispersion in the values of x of less than Δx=±0.7%. OA may also be used to determine lattice relaxation. The degree of uncertainty in the measurement of this parameter increases as lattice match is approached and decreases as the lattice relaxes. Our studies indicate that OA may be used as an independent technique to evaluate strain, alloy composition, and the degree of lattice relaxation in InxGa1−xAs epilayers.