The antioxidant toughening agent for the hydrocarbon based proton exchange polymer electrolyte membranes, cerium/organosiloxane polymer network (Ce/OSPN), is synthesized via sol-gel reaction. Ce/OSPN is introduced to the sulfonated poly(ether ether ketone) (SPEEK), a typical hydrocarbon type polymer electrolyte membrane, by formation of a semi-interpenetrating polymer network (semi-IPN) structure. As Ce/OSPN possesses superior properties to SPEEK in mechanical flexibility, proton conductivity, and oxidation stability than SPEEK, it resolves 3 inherent drawbacks of the pristine SPEEK membrane including (i) brittleness, (ii) low proton conductivity, and (iii) poor durability. Addition of 20 wt% Ce/OSPN (at Ce/silicon mol ratio = 0.10) enhances the elongation at break of the SPEEK membrane about twice. The power density of the MEA fabricated with the semi-IPN membrane is 208 mW cm(-2), which is much higher than that of the pristine SPEEK membrane, 165 mW cm(-2). The power density loss of the same semi-IPN membranes as determined by the Fenton's test is 4.8%, whereas those of pristine and semi-IPN membrane without cerium are 33.9% and 34.0%, respectively. This Ce/OSPN agent is expected to be applied to a variety of hydrocarbon based polymer electrolyte membranes.