The composition profile and lattice strain of GaAsSb grown on GaAs substrates by molecular beam epitaxy have been investigated using the Rutherford backscattering spectrometry (RBS) and X-ray reciprocal space mapping. Through RBS, we found that the Sb content in the layer increases from the interface to the top surface. The X-ray reciprocal space mapping shows that the GaAsSb lattice also gradually relaxes as the layer becomes thicker. The behavior of composition grading and gradual lattice relaxation is quite different from those of III-V ternary compounds with two group Ill atoms. (C) 2014 Elsevier B.V. All rights reserved.