Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earth's interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earth's interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressuretemperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earth's interior.diamond-anvil cell ͉ high-pressure research ͉ pressure calibration ͉ thermodynamics ͉ x-ray diffraction T he earth has a layered internal structure with distinct boundaries. The boundaries of the five main layers (the upper mantle, the transition zone, the lower mantle, the liquid outer core, and the solid inner core) are well defined by the observed seismic velocity discontinuities at depths of 400, 670, 2,891, and 5,149 km (corresponding to pressures of 13.4, 23.8, 135.8, and 328.9 GPa, respectively) in a global average preliminary reference earth model (PREM) (1). The interpretation of these discontinuities requires experimental investigations of earth materials at high pressure and temperature. The seismic discontinuities near 400 and 670 km depth are commonly associated with the mineralogical phase transformations of (Mg,Fe) 2 SiO 4 from ␣-olivine to ␤-phase (wadsleyite) and from ␥-spinel (ringwoodite) to (Mg,Fe)SiO 3 -perovskite plus (Mg,Fe)O-magnesiowüstite, respectively (2). With the rapid increase in the use of broadband seismometers and seismic arrays, seismologists have been able to determine the depths of the 400-and 670-km discontinuities and their lateral variation with increasingly finer resolutions (3). To correlate the observed seismic variability with the compositional and thermal variations in the mantle, we have to be able to determine mantle phase transitions with high accuracy, better than 1% in pressure determination (i.e., Ϯ0.25 GPa at 25 GPa). Similarly, it is critically dependent on the accuracy in pressure determination whether or not the recently discovered postperovskite transition (4, 5) indeed occurs at the base of the lower mantle and accounts for a number of seismic anomalies observed in the DЉ region. Because the DЉ layer is observed in a narrow depth int...

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