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High-pressure H 2 O polymorphs among which ice VI and ice VII are abundant in the interiors of large icy satellites and exo-planets. Knowledge of the elastic properties of these pure H 2 O ices at high-temperature and high-pressure is thus crucial to decipher the internal structure of icy bodies. In this study we assess for the first time the pressure-volume-temperature (PVT) relations of both polycrystalline pure ice VI and ice VII at high pressures and temperatures from 1 to 9 GPa and 300 to 450 K, respectively, by using in situ synchrotron X-ray diffraction. The PVT data are adjusted to a second-order Birch-Murnaghan equation of state and give V 0= 14.17 (2) cm 3 mol− 1, K 0= 14.05 (23) GPa, and α 0= 14.6 (14)× 10− 5 K− 1 for ice VI and V 0= 12.49 (1) cm 3 mol− 1, K 0= 20.15 (16) GPa, and α 0= 11.6 (5)× 10− 5 K− 1 for ice VII.