Terpolymers of ethylene, norbornene, and 5-exo norbornene methyl alcohol are prepared using Pd phosphine sulfonates as catalysts. The pendant hydroxyl groups are then transformed into thioacetate groups. Films cast from the resulting polymers are then oxidized by hydrogen peroxide. This green oxidation method is found to quantitatively transform thioacetate groups into sulfonic acids, leading to the formation of sulfonated hydrocarbon ionomers. These ionomers are thermally stable, exhibit increasing conductivity up to 110 C, and have a low water uptake, indicating that these materials are potentially interesting candidates for the preparation of fuel cell membranes. remain a limiting element in the development of this technology. Currently, the most plausible candidate for a massproduced PEM is NafionV R , a perfluorinated polymer with sulfonic acid as pendant group. 1 An unique feature of Nafion is the microphase separation between the hydrophobic backbone and the hydrated sulfonic acid domains, resulting in the formation of water channels for the transport of protons.2 However, this fluorinated ionomer is expensive and its production requires stringent conditions. Additional drawbacks of Nafion include high methanol permeability, low conductivity at temperatures higher than 100 C and at low relative humidity, and poor mechanical stability at elevated temperature. The search for better PEMs for proton exchange fuel cells remains a challenge for scientists despite a plethora of articles regarding this issue.