Reciprocal space mapping by high-resolution X-ray diffraction is a powerful method for detailed structural characterization of imperfect crystalline layers. The application of a relaxation line model for the interpretation of reciprocal space maps will be demonstrated for epitaxially grown Il-VI heterostructures. This model enables one to distinguish between a gradient of the strain and of the chemical composition in epitaxial layers. In addition, the use of a relaxation line model in describing thermoelastic strain in heterostructures is shown for HgSe layers on ZnTe buffers grown on GaAs(001) substrates. The degree of thermoelastic strain can be influenced by either the growth temperature or by the temperature during characterization. The former was investigated.