The lattice parameters of β phase <(Mg0.84Fe0.16)2SiO4> have been determined by X ray diffraction using synchrotron radiation under simultaneous high-pressure and high-temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao-Bell type diamond anvil cell using neon gas as pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high-pressure calibrant at high temperatures. Using 4.0 for (∂KT/∂P)T, we found that the 300-K isothermal bulk modulus, KT0, and its temperature derivative at constant pressure, (∂KT/∂T)P, are 174(¿3) GPa and -2.7(¿0.3)¿10-2 GPa/K, respectively. The value of the Anderson-Gr¿neisen parameter ΔT of the β phase is 5.1(¿0.8) above the Debye temperature. The experimental results provide direct measurements of the pressure effect on the thermal expansivity and the temperature dependence of bulk modulus for the β phase. ¿ American Geophysical Union 1992