The equations of state of dense hydrous magnesium silicates (DHMS), determined from high-pressure single-crystal X-ray diffraction are reviewed, including hydroxylchondrodite, hydroxylclinohumite, phase A, phase B (anhydrous and hydrous), superhydrous phase B and phase E. The phases along the forsterite–brucite join, Mg2SiO4–Mg(OH)2, display near (increasing) linearity in compressibility with respect to water content and increasing bulk moduli (KT) with density. Such trends allow prediction of the as yet unknown bulk moduli of phases such as OH-Mg norbergite. The addition of water also reduces the bulk modulus of the B-phases and the anisotropy observed in axial compression. The alternating layers of octahedra and octahedra + tetrahedra completely control compression of the B phases, with the stacking direction becoming more compressible with addition of water. The enigmatic Phase E has the highest KT′ yet measured for a hydrous silicate and one of the lowest KT. In contrast with other DHMS, Phase E is only slightly anisotropic in axial compression and we attribute this to the role of the intralayer cations in the structure. The degree of hydration and the vacancy concentration appear to be coupled in Phase E.