AbstractIn order to study the relationship between the properties of proton electrolyte membranes (PEMs), obtained through standard characterization methods, and the direct methanol fuel cell (DMFC) performance, inorganic–organic hybrid membranes, modified via in situ hydrolysis, were used in a membrane electrolyte assembly (MEA) for DMFC application. The membranes, the characterization of which was performed in the previous paper of this series, were based on sulfonated poly(ether ether ketone) (sPEEK) with a sulfonation degree (SD) of 87% and were loaded with different amounts of zirconium oxide (5.0, 7.5, 10.0, 12.5 wt.%). The standard characterization methods applied were impedance spectroscopy (proton conductivity), water uptake, and pervaporation (permeability to methanol). The MEAs were characterized investigating the DMFC current–voltage polarization curves, constant voltage current (CV, 35 mV), and open-circuit voltage (OCV). The fuel cell ohmic resistance (null phase angle impedance, NPAI) and CO2 concentration in the cathode outlet were also measured. The characterization results show that the incorporation of the inorganic oxide in the polymer network decreases the DMFC current density for CV experiments, CO2 concentration in the cathode outlet for both OCV and CV experiments and, finally, the maximum power density output. The opposite effectwas verified in terms of the NPAI (ohmic resistance) for both OCV and CV experiments. A good agreement was found between the studied DMFC performance parameters and the characterization results evaluated by impedance spectroscopy, water uptake and pervaporation experiments.