We report a comparative study of transport and thermodynamic properties of single-crystal and polycrystalline samples of the ionic salt CsH5 (PO4)(2) possessing a peculiar three-dimensional hydrogen-bond network. The observed potential of electrolyte decomposition approximate to 1.3 V indicates that the main charge carriers in this salt are protons. However, in spite of the high proton concentration, the conductivity appears to be rather low with a high apparent activation energy E-a approximate to 2 eV, implying that protons are strongly bound. The transport anisotropy though is not large, correlates with the crystal structure: the highest conductivity is found in the [001] direction (sigma(130 degrees C) similar to 5.6 x 10(-6) S cm(-1)) while the minimal conductivity is in the [100] direction (sigma(130 degrees C) similar to 10(-6) S cm(-1)). The conductivity of polycrystalline samples appears to exceed the bulk one by 1-3 orders of magnitude with a concomitant decrease of the activation energy (E-a approximate to 1.05 eV), which indicates that a pseudo-liquid layer with a high proton mobility is formed at the surface of grains. Infrared and Raman spectroscopy used to study the dynamics of the hydrogen-bond system in single-crystal and polycrystalline samples have confirmed the formation of such a modified surface layer in the latter. However, no bulk phase transition into the superionic disordered phase is observed in CsH5(PO4)(2) up to the melting point T-melt similar to 151.6 degrees C, in contrast to its closest relative compound CsH2PO4. (c) 2006 Elsevier B.V. All rights reserved.