The pressure dependence of the structural parameters and bonding characteristics of Ca(OH)2-II have been investigated by density functional theory calculations up to 9 GPa. The bulk modulus of this phase is predicted to be B 47(2) GPa. The pressure-induced transition between portlandite and Ca(OH)2-II is computed to occur at 4.4 GPa in the athermal limit, slightly lower than the experimental value of 5.7(4) GPa determined at 773 K. The enthalpy difference of the two phases at ambient pressure is calculated to be −15 kJ/mol, in good agreement with a value of −10.4(5) kJ/mol derived earlier from DSC data. The computed change of the molar volumes at the phase transition is 11%, in good agreement with a value of ΔV = 12% derived in high-pressure/high-temperature experiments. The predicted pressure-induced structural changes and the bond population analyses of the phases at high pressures imply that there are no significant changes in the hydrogen bonds and hence that this transition is not driven by pressure-induced hydrogen bonding.