In-rich and Ga-rich GaInP films were intentionally grown on (0 0 1) GaAs substrates by low-pressure MOCVD to investigate the effect of lattice strain on composition. High-resolution X-ray diffraction (HRXRD) measurement showed that a GaInP single layer exhibits a double-diffracted peak phenomenon. Such a double peak represents a composition separation in the grown film, resulting in two absorption cutoff energies in optical absorption analysis. Cross-sectional transmission electron microscopic (TEM) observation confirmed the composition separation in an In-rich GaInP film. Furthermore, the composition separation amount of a Ga-rich GaInP film after substrate removal was found to be similar to 0.5%, which reflects the actual effect of lattice strain on composition during growth stage. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.