X‐ray diffraction has been used to measure the residual strain in In1−xAlxSb (x=0.15–0.64) layers of thicknessh=0.1 &mgr;m, grown epitaxially on (100) InSb substrates. The results for this tensile‐strained system are compared with a well‐studied compressive‐strain system, viz., InGaAs/GaAs. Experimental evidence is presented which suggests that the strain relaxation mechanism in this system is different from that in InGaAs/GaAs. This includes an observed critical layer thickness (hc) which is about five times thehcgiven by the Matthews and Blakeslee [J. Cryst. Growth27, 118 (1974)] type models and a measured relaxation coefficient which is quasilinear inh/hcfor 5<h/hc<35. The results have also been compared with predictions of the Dodson–Tsao (DT) model [Appl. Phys. Lett.51, 1710 (1987)] which fits well the quasilinear behavior of the relaxation coefficient. In addition, it is argued that the DT equation provides a generic model that is not restricted to dislocation‐mediated strain relief. Other possible mechanisms of strain relaxation in InAlSb/InSb are discussed. ©1995 American Institute of Physics.