Several GaSb-based infrared devices require the growth of dislocation-free AlSb layers lattice matched to the GaSb substrate. In this work, we characterize the strain relaxation of AlSb films grown on GaSb by molecular beam epitaxy. We grew several AlSb films, ranging in thickness from 40 Angstrom to 2 mum, on (0 0 1)-GaSb substrates. Despite a Matthews-Blakeslee critical thickness of approximately 200, our results indicate that the AlSb remains highly strained for film thicknesses above 1000 Angstrom and even retains significant strain (similar to18%) for 2 mum AlSb films. We demonstrate that the simple use of Bragg's Law for predicting the strain from X-ray rocking curve data is insufficient for the very thin films. (C) 2002 Elsevier Science Ltd. All rights reserved.