We report the first measurement of a complete set of thermoelastic equation of state of a clinopyroxene mineral. We have conducted an in situ synchrotron x-ray diffraction study of jadeite at simultaneous high pressures and high temperatures. A modified Rietveld profile refinement program has been applied to refine the diffraction spectra of low symmetry and multiple phases observed in energy dispersive mode. Unit cell volumes, measured up to 8.2 GPa and 1280 K, are fitted to a modified high-temperature Birch-Murnaghan equation of state. The derived thermoelastic parameters of the jadeite are: bulk modulus K'=125 GPa with assumed pressure derivative of bulk modulus K˙=∂K/∂P=5.0, temperature derivative of bulk modulus K=∂K/∂T=-1.65¿10-2 GPa K-1, and volumetric thermal expansivity &agr;=a+bT with values of a=2.56¿10-5K-1 and b=0.26¿10-8K-2. We also derived thermal Gr¿neisen parameter &ggr;th=1.06 for ambient conditions; Anderson-Gr¿neisen parameter ΔTo=5.02, and pressure derivative of thermal expansion ¿&agr;/¿P=-1.06¿10-6 K-1 GPa-1. From the P-V-T data and the thermoelastic equation of state, thermal expansions at five constant pressures of 1.0, 2.5, 4.0, 5.5, and 7.5 GPa are calculated. The derived pressure dependence of thermal expansion is: Δ&agr;/ΔP=-0.97¿10-6K-1 GPa-1, in good agreement with the thermodynamic relations.¿ 1997 American Geophysical Union