Lattice strain relaxation in InAs/GaSb heterostructures was investigated by x-ray diffraction. Two types of structures, grown by molecular beam epitaxy, are compared. In the first, GaSb buffer layers were grown on GaAs substrates, followed by 0.05–1.0 &mgr;m thick InAs layers. In the second, InAs layers were grown directly on GaSb substrates. For a given thickness, the InAs layers retain significantly more strain when grown on GaSb substrates, reflecting the lower threading dislocation density in the GaSb substrates relative to the GaSb buffer layers grown on GaAs.