The composition of several Ga1-xAlxSb epitaxial layers of different thicknesses grown by molecular beam epitaxy on GaSb with x ranging between 0.1 and 0.8, has been obtained independently by high resolution x-ray diffraction, Rutherford backscattering spectrometry, and reflection high-energy electron diffraction. From the comparison between the results obtained by the different experimental methods, it has been possible to point out that the lattice constant of the layer increases nonlinearly with the Al content. A comparison with theoretical models has been done. A phenomenological equation has been derived for a correct analysis of the x-ray results. © 1999 American Institute of Physics.