Proton conductivity, thermal and structural properties of (1 - x)CsHSO4-xKH(2)PO(4) mixed salts in a wide range of compositions (x=0.05-0.95) has been studied. Two new phases with different crystalline structures, transport and thermal properties have been found in the system. New highly conductive phase with a Cs-3(HSO4)(2)(H2PO4) (C2/c) structure is formed at x = 0.05-0.5 with the conductivity at low-temperatures more than two orders of magnitude higher than that of the source salts. At higher x (x=0.6-0.9) another phase with the CsH5(PO4)(2) (P2(1)/c) crystalline structure is formed. This phase has lower thermal stability, relatively low proton conductivity and high activation energy. (C) 2012 Elsevier B.V. All rights reserved.