A clear pattern of systematic error is found in thermal expansion values for Al, Ag, and Au commonly recommended and used as internal references for temperature determination in x‐ray diffraction. Evidence comes from a critical analysis of the literature, including several studies of temperature measurement errors in powder cameras, and from new calibration of a thermometer used previously in precise lattice and length expansion studies. Necessary changes in the commonly accepted values for expansion of Pt and MgO are implied.