Interfacial elastic strain induced by the lattice parameter mismatch between epilayer and substrate results in significant energy–band‐gap shifts for III‐V alloys. The epilayers used in this study are GaxIn1−xAs on (100) InP and GaxIn1−xP on (100) GaAs prepared by organometallic vapor phase epitaxy. For layer thicknesses between 1 and 1.5 &mgr;m, and &Dgr;as.f./a0≤3.5×10−3the misfit strain is assumed to be accommodated elastically. The energy–band‐gap shifts are determined by comparing the photoluminescence peak energies of the epilayers with the best experimental relation of band gap versus composition for unstrained layers. A calculation of the energy–band‐gap shift due to biaxial stress made for GaxIn1−xAs is found to agree with the photoluminescence measurements. In addition, a comparison of the energy–band‐gap shift for GaxIn1−xP shows a clearly different dependency for tensile and compressive strain, in good agreement with calculated results.