Electrical conductivity, differential scanning calorimetry, and X-ray diffraction measurements were performed on a pentacesium trihydrogen tetrasulfate, Cs5H3(SO4)(4), crystal. The transition entropy at a superionic phase transition and the activation energy of proton migrations in the superionic phase were determined to be 58.2 J K-1 mol(-1) and 0.48 eV, respectively. The crystal structure Of Cs5H3(SO4)(4) at room temperature was refined. The electrical conduction in Cs5H3(SO4)(4) was discussed with the refined structure.