The influence of the small deviations of Cs:P ratio in CsH2PO4 has been firstly investigated. The influence of small amounts of H3PO4 or CsH5(PO4)2 additives (xmole = 0.002–0.05) on the electrotransport, structural and thermal properties of CsH2PO4 has been investigated by impedance and IR spectroscopy, XRD and DSC methods. Effect of the acidic centers on the proton conductivity was discovered. The doping of CsH2PO4 up to x ≤ 0.03 is virtually invisible effect for X-ray diffraction and IR methods, but influences on the superionic phase transition, thermodynamic characteristics and increases significantly the low-temperature conductivity. The superionic phase transition becomes more diffusive and moves to the lower temperatures up to ~ 200 °C. According to XRD the formation of (1 − x)CsH2PO4 − xCsH5(PO4)2 composite electrolyte based on two ionic salts was observed for both types of dopants. The addition of CsH5(PO4)2 (0.2 mol%) increases a low temperature conductivity of CsH2PO4 by more than one order of magnitude and decreases the activation energy of conductivity at the same time. The low-temperature conductivity enhances up to σ160°C ~ 10− 3 S·cm− 1 with Ea = 0.65 eV at x = 0.05. The doping of CsH2PO4 up to 5 mol% does not influence significantly on the high-temperature conductivity.