High resolution diffraction measurements of the strained lattice unit of HgCdTe and CdZnTe have been performed at temperatures varying from room temperature to 300 degrees C and for different lattice mismatch between substrate and layer. This investigation makes possible the determination of the coefficients of thermal expansion (CTE) and the evolution of the HgCdTe film stress during the thermal cycles. It is found that the CTE is linear with the zinc fraction for CdZnTe while it can be described by a parabolic variation as a function of the cadmium fraction for HgCdTe. The temperature evolution of the stress is found to be dictated by the CTE difference between substrate and layer up to a temperature of 150 C above which the HgCdTe layer partially relaxes. The evolution of the stress with lattice mismatch enables the determination of the onsets for plastic relaxation for both tensile and compressive stress. (C) 2013 Elsevier B.V. All rights reserved.