The thermal coefficient of expansion of AgCl has been measured as a function of temperature from 120 to 710°K (melting point = 728°K) by means of x-ray diffraction from small single crystals. Above 300°K the results agree well with the dilatometric measurements reported by Strelkow. Such agreement indicates that the concentration of Schottky defects in AgCl is not large enough to influence significantly the thermal expansion below 710°K. The thermal expansion for the entire temperature range is described rather well by Grüneisen's theory, (1) if it is assumed that thermally generated Frenkel defects contribute significantly to the high-temperature thermal expansion, and (2) if two parameters in the theory are chosen to give a good fit to the low-temperature (T<300°K) x-ray data. Attempts to determine the activation energy of the Frenkel defects from comparison of the thermal expansion data with theory indicate that certain constants of the theory are probably temperature dependent. Below 300°K the x-ray results differ significantly from the dilatometric results reported by Sreedhar. Low-temperature x-ray measurements of the thermal expansion of Al are, therefore, included and compared with existing data in the literature to demonstrate the validity of our experimental technique. The especially convenient experimental technique used is described.