The phase composition, electrotransport properties, and spectral characteristics of the (1 − x)CsH2PO4–xButvar system (х = 0–0.2 wt%) were investigated in details. Butvar was proved to be a chemically suitable matrix for CsH2PO4 with a good adhesion to acid salt permitting the formation of flexible membrane. The chemical interaction of organic matrix with acid salt was not observed. A new method of “polymer-CsH2PO4” composites preparation through alcohol solutions of initial components was proposed. The flexible and thin (~ 200 μm) proton membranes with uniform salt distribution were obtained. Composite polymer electrolytes are characterized by high proton conductivity values (~ 5 × 10−3 S/cm) at 240 °C and are stable under conditions of high humidity (PH2O ~ 0.3 atm.).