Phase composition and phase transitions, spectral characteristics, thermal and transport properties of the (1-x) RbH2PO4 - xRb(2)HPO(4)center dot yH(2)O system at the wide range of x = 0-1 were investigated. No solid solutions were observed at the system. A monophasic region at x = 0.25 with the compound Rb5H7(PO4)(4) was found. The thermodynamic and transport properties of Rb5H7(PO4)(4) were studied in detail. The other compositions correspond to diphasic mixtures of two salts: RbH2PO4 and Rb5H7(PO4)(4) (x = 0-0.2), or Rb2HPO4 center dot yH(2)O and Rb5H7(PO4)(4) (at x higher 0.25). The sharp increase of proton conductivity up to 3.10(-2) S/cm under highly humidified conditions was found for x = 0.1-0.25 at temperatures > 240 degrees C can be associated with the super ionic phase transitions of RbH2PO4 and Rb5H7(PO4)(4). A weak composite effect with a slight increase of conductivity (about one order of magnitude) at low temperatures compared to those for the pure salts RbH2PO4 and Rb2HPO4 center dot 2H(2)O was shown for all samples.