Raman and powder X-ray measurements were obtained up to 4 GPa for phases II and III of Rb3H(SO4)(2). Geometry optimization by the DFT methods was carried out using the data of powder X-ray measurements. The pressure dependence of the strength of the hydrogen bond corresponded well to the change in proton conductivity. The crystal structure calculated by the geometry optimization indicated an exchange of the counterpart of the dimer at the transition. This caused a temporal increase in the hydrogen bond length and an increase in proton conductivity at the II-III transition. (C) 2010 Elsevier B.V. All rights reserved.