Abstract The dynamic compression of both vitreous and rutile-type GeO 2 has been studied throughout the pressure range 0–160 GPa (1.6 Mbar). At sufficiently high pressures (respectively >35 GPa and >70 GPa) both materials attain densities greater than those expected for the rutile phase at the same pressure. These results may be explained in terms of transformation of both vitreous and rutile-type GeO 2 to a common high-pressure phase or state with a zero-pressure density ∼5% greater than that of the rutile polymorph. The vastly different thermal regimes associated with the two Hugoniots allow important deductions to be made concerning the elastic and thermodynamic properties of shocked germanium dioxide. In particular, an effective Grüneisen parameter of 1.24 ± 0.1 is required at a density of 7.4 g/cm 3. Inconsistency between this value and those calculated from the Vashchenko-Zubarev model suggests that the latter may not provide an adequate description of the thermal properties of close-packed germanates and silicates.