The crystalline quality of InGaAsP deposited via gas-source molecular beam epitaxy on rectangular-patterned InP substrates has been investigated. Triple axis x-ray diffractometry measurements of InGaAsP deposited on planar InP substrates confirm that the pseudomorphic epilayer composition is uniform and of high-crystalline quality. Triple axis x-ray diffraction.analysis of InGaAsP deposited on rectangular-patterned substrates reveals that the crystalline quality of the epilayer is not compromised by the presence of the grating. Symmetric [(004)] and asymmetric [(224)] reciprocal space maps of overgrown rectangular gratings indicate that both the InP.and InGaAsP within the grating region exhibit an overall orthorhombic lattice distortion : Comparison of (004) diffraction scans with dynamical computer simulations suggests that the distortions are not related to detectable compositional grating. The magnitude of the orthorhombic strain, introduced by the rectangular-patterned grating, will alter the refractive index of both materials by amounts that are not expected to compromise the operation of optical devices.