Equation of state properties of ice VII and fluid H2O at high pressures and temperatures have been studied experimentally from 6 to 20 GPa and 300–700 K. The techniques involve direct measurements of the unit‐cell volume of the solid using synchrotron x‐ray diffraction with an externally heated diamond–anvil cell. The pressure dependencies of the volume and bulk modulus of ice VII at room temperature are in good agreement with previous synchrotron x‐ray studies. The thermal expansivity was determined as a function of pressure and the results fit to a newly proposed phenomenological relation and to a Mie–Grüneisen equation of state formalism. The onset of melting of ice VII was determined directly by x‐ray diffraction at a series of pressures and found to be in accord with previous volumetric determinations. Thermodynamic calculations based on the new data are performed to evaluate the range of validity of previously proposed equations of state for fluid water derived from static and shock‐wave compression experiments and from simulations.

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