The influence of the preparation conditions and metaphosphate secondary phase on the proton conduction and chemical stability of the single and polycrystalline CsH2PO4 samples was studied by the ac admittance technique. The relation between frequency dependences of the admittance plots and adsorbed water as well as a content of the metaphosphate phase was determined. It was shown that in ambient conditions the thermal decomposition reactions start on the sample's surface at temperatures above specific point, T-I =425 K, and the rate of the decomposition reactions on the interfaces is strongly influenced by the sample's quality. It is suggested that this reaction is initiated by the melting of the thin layer of CsH5 (PO4)(2) on the surface of CsH2PO4. The variation of the contributions of the intergrain and electrode-proton conductor boundaries to the total admittance with the temperature was studied for polycrystalline CDP samples and obtained data were compared with those for nominally pure CDP Single crystal samples. (C) 2007 Elsevier B.V. All rights reserved.