Thermally induced strain is used to induce plastic strain relaxation in a 280 nm thick ZnSe layer grown on a GaAs substrate. Changes in the crystalline quality in dependence on temperature an investigated in situ by high-resolution X-ray diffraction. Reciprocal space maps and triple-axis rocking curves are performed to measure the diffuse scattered intensity around the ZnSe Bragg peak. A striking change in the diffuse scattering is observed when heating the sample above growth temperature up to 600 K. From the diffuse scattered intensity the misfit dislocation density in the as-grown and the thermally treated layer is estimated to be 2 x 10(3) and (1-2)x 10(4) cm(-1), respectively. These results are in agreement with dislocation densities estimated from the lateral lattice mismatch.