Lattice strain relaxation has been studied in the Ga1-xAlxSb/GaSb system by analyzing layers with thicknesses ranging between 0.1 and 6 μm and with Al concentration x=0.4. The samples have been grown by molecular beam epitaxy at 550 °C on (001) oriented undoped GaSb substrates. The heterostructures were investigated by a high resolution x-ray diffraction method. The experimental critical thickness for the strain relaxation has been found to be tc≈ 310 nm. The measured residual strain Єres shows a t-0.47 dependence on the layer thickness t. This result is in agreement with the theoretical predictions based on the energy balance model, and cannot be described in the frame of the equilibrium theories which yield Єres∝t-1. The values of Єres have been corrected by the so-called thermal misfit (ΔT) for epitaxial systems with different thermal expansion coefficients. For the Ga0.6Al0.4Sb/GaSb epitaxial system grown at TG=550 °C, the value ΔT=-3.28×10-4 has been calculated from the literature data.© 1997 American Institute of Physics.