A series of proton exchange membranes (PEMs) of sulfonated poly(2,6‐dimethyl‐1,4‐phenylene oxide) (SPPO) with different sulfonated graphene oxide (SGO) (0, 0.25, 0.5, 0.75, 1.0, 1.5%) were prepared for possible application in Iron‐air/flow battery. The presence of SGO in the membrane improved the water uptake of all membranes prepared and decreased the swelling ratio. The SPPO/1% w/w SGO showed good properties with higher water uptake of 44.1%, water retention of 42.48% and swelling ratio of 21.4% at 25°C. However, the 0.75% SGO exhibited the highest proton conductivity of 0.98 S/cm and better dispersion, elemental distribution, and morphology. In addition, SGO improved the thermal stability of all composite membranes. The chemical stability of all membranes remained good after 7 days in a 1 M HCL solution. The SPPO/0.75%w/w SGO showed increased proton conductivity when degradation properties were evaluated. Compared to Nafion 117, all composites showed lower iron crossover. Nafion had a high permeability of 3.03 × 10−7 cm−2/s, which is 75% higher than the 7.6 × 10−8 cm−2/s of SPPO/SGO‐1.5% and 52% higher than the 1.46 × 10−7 cm−2/s of SPPO/SGO‐0.75%. The results indicate that SGO is a good inorganic filler for the composite proton exchange membrane and stable enough for possible application in an iron‐air/flow battery.