The electrotransport, structural and thermodynamic properties of CsH2PO4 with small amount of Cs2HPO4·2H2O (mole fraction, x = 0.01–0.1) has been investigated by the impedance, IR spectroscopy, X-ray diffraction and differential scanning calorimetric methods. The addition of Cs2HPO4·2H2O up to mole fraction x ≤ 0.03 was shown to increase markedly the low-temperature conductivity and change the thermodynamic characteristics of cesium dihydrogen phosphate. The low-temperature conductivity enhances up to σ150°C ~2 · 10−3 S·cm−1 for x = 0.1 and the activation energy of conductivity decreases to Ea = 0.56 eV. The superionic phase transition becomes more diffusive and almost disappears at x = 0.1. The high-temperature conductivity doesn't change significantly at the doping of CsH2PO4 up to x = 0.1. While the small deviation of proton content in CsH2PO4 shows a weak effect for XRD and IR spectroscopy. According to X-ray diffraction patterns the Cs1+xH2−xPO4 solid solutions were not observed and the formation of (1-2x)CsH2PO4-xCs3(H1.5PO4)2 composites took place. The effect of the deficiency of protons on the conductivity of CsH2PO4 in the low humidity conditions was discovered firstly and is of scientific and practice importance.