GaAsSb ternary epitaxial layers were grown on GaAs (001) substrate in various Sb-4/As-2 flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs1-ySby were inferred using high-resolution X-ray symmetric (004) and asymmetric (224) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb-4 and As-2 under the assumption that one incident Sb-4 molecule produces one active Sb-2 molecule. It is inferred that the activation energy of Sb-4 dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data. (C) 2004 Elsevier B.V. All rights reserved.