AbstractA new proton‐conducting polymer electrolyte—poly‐(benzylsulfonic acid)siloxane (PBSS)—has been prepared by sol–gel process. Both the hydrolysis and the condensation reactions of triethoxybenzylsilane have been studied under different catalysis conditions. The basic material—polybenzylsiloxane (PBS)—is always soluble, no matter what the experimental conditions, reaching an average molecular weight—Mw—of 20,000 and a polydispersity index close to 5. Good yields of a water‐soluble polyelectrolyte—PBSS–thermally stable up to 300°C in air according to thermogravimetric analysis (TGA), and for which the exchange capacity is close to 4.4, can be obtained by sulfonation of PBS. Conductivities ranging from 2 × 10−3to 10−2S/cm at room temperature have been determined by a.c. impedance spectroscopy. These high conductivities, as well as the tensile strength of the PBSS, allow its use in se