Gallium arsenate dihydrate is a maniber of a class of isostructural compounds, with the general formula M3+AsO4 center dot 2H(2)O (M3+ = Fe, Al, In, or Ga.), which are being considered as potential solid-state storage media for the sequestration of toxic arsenic cations. We report the first high-pressure structural analysis of a metal arsenate dihydrate, namely, GaASO(4)center dot 2H(2)O. This compound crystallizes in the orthorhombic space group Pbca with 2 = 8. ACcurate unit cell parameters as a function ofpressure were obtained by high-pressure single-crystal X-ray diffraction, and a bulk modulus of 51.1(3) GPa for GaAsO4 center dot 2H(2)O was determined from a third-order Birch Murnaghan equation of state fit to the P-V data. Assessment of the pressure dependencies of the unit cell lengths showed that the compressibility of the structure along the axial directions ancreaseS in the order of [010] < [100] < [001]. This order was found to correlate well With the proposed compression mechanism for GaAsO4 center dot 2H(2)O, which involves deformation of the internal channel void spaces of the polyhedral helices that lie parallel to the [010] direction, and increased distortion of the GaO6 octahedra. The findings of the high-pressure diffraction experiment were further supported by the results from variable-pressure Raman analysis of GaAsO4 center dot 2H(2)O. Moreover, we propose a revised and more complex model for the hydrogen-bonding scheme in GaAsO4 center dot 2H(2)O, and on the basis of this revision, we reassigned the peaks in the OH stretching regions of previously published Raman spectra of this compound.