Abstract. Twenty-one energy-dispersive X-ray diffraction spectra for fayalite at 400~ constitute the basis for an elevated-temperature static compression isotherm for this important silicate mineral. A Murnaghan regression of the resulting molar volumes yields 103.8 GPa and 7.1 for the 400 ~ C, room-pressure values of the isothermal bulk modulus (Ko) and its first pressure derivative (K;), respectively. When compared to the room-temperature static compression isotherm of Yagi et al. (1975), our 400 ~ C value for Ko yields -5.4