Structural, transport and thermal properties of the Cs-1 (-) xMxH2PO4 (M = Na, K, Rb) systems were studied in a wide range of compositions (x = 0-0.9). The solid solutions are formed in all Cs-1 (-) xMxH2PO4 systems. The cell parameters in double salts decrease compared to CsH2PO4. The range of existence of Cs-1 (-) xMxH2PO4 solid solutions significantly differs for different cations, x = 0-0.9 for M = Rb+, x = 0-0.3 for K+ and Na+. Proton conductivity of low-temperature phase of double salts increases by 1-3 orders of magnitude depending significantly on the composition and size of introduced metal ions, whereas proton conductivity of high-temperature phase decreases no more than 1.5-2 times. The temperature of superionic phase transition increases for the Cs-1 (-) xRbxH2PO4 systems (about <= 20 degrees C) and decreases for Cs-1 (-) xKxH2PO4. (C) 2012 Elsevier B.V. All rights reserved.