The effect of rapid thermal annealing on a 10-layer InAs/In0.15Ga0.85As dots-in-a-well (DWELL) heterostructure was studied using double crystal x-ray diffraction (DCXRD) and photoluminescence (PL). From the x-ray rocking curves obtained for symmetric (004) and asymmetric (224) scans, the change in the in-plane and out-of-plane lattice constant and average composition in the DWELL structure were calculated. Thermally induced strain relaxation, which leads to an enhanced In/Ga interdiffusion preferentially along the growth direction, is believed to be the main mechanism for the changes in the structural and optical properties of the sample. Excellent correlation was observed between the PL and the DCXRD measurements. © 2002 American Institute of Physics.