Ikaite (CaCO3·6H2O), which forms in cold carbonate-rich marine environments, also crystallizes from calcite under aqueous conditions above ~0.5 GPa at room temperature. Using synchrotron X-ray powder diffraction, measurements have been made of pressure-induced changes in unit-cell dimensions of ikaite contained in a diamond anvil cell. Ikaite shows anisotropic compressibility along the crystallographic axes in the order a > c > b up to 4 GPa. Comparison with other phases shows the relative volume compressibility of ikaite to be greater than that of gypsum (CaSO4.2H2O), calcite and aragonite. The volume of ikaite is less than that occupied by equivalent volumes of CaCO3 (calcite/aragonite) + 6H2O at all pressures below the freezing point (H2O(l) to ice VI). The bulk modulus of ikaite has been obtained from a fit to the Vinet equation of state, giving V0 = 760.3 ± 0.1 Å3, K0 = 21.3 ± 1.4 GPa, and K0′ = 11.7 ± 1.7. Dissolution of CaCO3 in H2O (l) at high pressure resulting in crystallization of CaCO3·6H2O suggests equilibrium behaviour between a carbonate-rich fluid species and the condensed hydrated calcium carbonate. The potential for ikaite as a candidate phase for water transport in cold subduction zones is considered.