Abstract We have developed preparation protocol of practically large size self-humidifying polymer electrolyte membranes (PEMs) with highly dispersed nanometer-sized Pt and/or SiO2 for fuel cells. The Pt particles were expected to catalyze the recombination of H2 and O2, leading to a suppression of the chemical short-circuit reaction at the electrodes, while the SiO2 particles were expected to adsorb the water produced at the Pt particles together with that produced at the cathode reaction. Stable SiO2 particles were formed in a commercial PEM (Nafion®112) via in situ sol–gel reactions at 70 °C. It was found by SAXS that the hydrophilic cluster size increased by water adsorbed SiO2, which may contribute to the increase in the proton conductivity once SiO2 adsorbed water. Pt particles were uniformly dispersed in a Na+-form normal-PEM or SiO2-PEM by an ion-exchange reaction with [Pt(NH3)4]Cl2, followed by a reduction with 1-pentanol at 125 °C. The newly prepared Pt-SiO2-PEM was found to perform a self-humidifying operation in a standard-size PEFC (25 cm2 electrode area) with H2 and O2 humidified at 30 °C. The performance of the Pt-SiO2-PEM cell operated with the low humidity reactant gases was as high as the normal-PEM cell fully humidified, because the ohmic resistance of the former cell was as low as the latter cell.