Phase composition and phase transitions, spectral characteristics, thermal and transport properties of the (1-x)RbH2PO4 – xRb2HPO4 · yH2O system at the wide range of х = 0–1 were investigated. No solid solutions were observed at the system. A monophasic region at х = 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 (х = 0–0.2), or Rb2HPO4·yH2O and Rb5H7(PO4)4 (at х higher 0.25). The sharp increase of proton conductivity up to 3·10−2 S/cm under highly humidified conditions was found for х = 0.1–0.25 at temperatures >240 °С can be associated with the superionic 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 Rb2HPO4·2H2O was shown for all samples.