The objective of this investigation is to study silica-doped/sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) composite membranes for operation in hydrogen/oxygen proton-exchange membrane fuel cells ranging from room temperature (RT) up to 120 °C. The sulfonated PPO composite membranes were prepared using a sol–gel process employing reaction with tetra-ethoxysilane (TEOS) followed by heat treatment at 60, 90, and 120 °C, respectively. The presence of silicon oxide in the composite membranes was evaluated using FTIR spectroscopy, while thermal properties were studied using thermal gravimetric analysis-differential scanning calorimetric (TGA-DSC) measurements. Additionally, ion exchange capacity, water uptake, and proton conductivity characterizations were also carried out. It was observed that water uptake for 75% PPO sulfonated composite membrane treated at 120 °C is higher than that of NafionTM membrane and the proton conductivity value measured at 120 °C is 0.35·10−1 S/cm. Therefore, the composite membranes are potentially suitable for high temperature fuel cell applications.