An ab initio DFT-based study of the electronic band structure, density of states. and Mulliken charges of constituent ions of CsH5(PO4)(2) crystal has been performed for different unit cell volumes corresponding to the external pressure from the range 2-6 GPa. The crystal reveals a relatively small dispersion of the band energy E(K) and a relatively wide indirect band gap E-g = 7.8 eV. The bulk modulus B and all components of the elastic tensor cold are calculated. Results of the electron energy changes of CsH5(PO4)(2) upon hydrogen atom displacements agree with the anisotropy of the experimental proton conductivity of the crystal. (C) 2011 Elsevier B.V. All rights reserved.