in order to clarify critical thickness issues for epitaxial layers of CdxHg1-xTe on Cd1-yZnyTe substrates, it is desirable to have accurate data for the difference in thermal expansion between these materials. Based on molecular beam epitaxial samples, we have measured thermal expansion of the two materials simultaneously, over the temperature range 25-110degreesC, using high-resolution X-ray diffraction. Measured CdHgTe values are corrected for effects of layer strain. Data cover the composition ranges x = 0-0.7 and y = 0-0.05. Thermal expansion of narrow gap CdHgTe is found to exceed that of lattice matched CdZnTe substrates by approximately 0.3 ppm/K. This can quantitatively account for a previously reported asymmetry in layer relaxation. (C) 2002 Published by Elsevier Science B.V.