Ab initio simulations of water adsorbed in the microporous silicoaluminum phosphate HSAPO-34 are reported. A neutron diffraction study by L. Smith et al., Science 1996, 271, 799, showed that both H2O and H3O+ are present when this solid acid is loaded with water. The ab initio simulations support this; however, new details are revealed which are not accessible by diffraction experiments averaging over all sites. The acid-base reaction requires at least three water molecules per two nearby acidic sites in the micropores of the solid and leads to a protonated water cluster, H3O+(H2O)(2).