High-pressure, high-temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation-of-state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298-K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero-pressure bulk modulus KT0=261(¿4) GPa with its pressure derivative KT0'=4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P-T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is -6.3(¿0.5)¿10-2 GPa/K. analyses of the high-temperature data give a value for the Anderson-Gr¿neisen parameter ΔT of 6.5--7.5, which is significantly higher than that used in recent lower mantle models. ¿ American Geophysical Union 1991