Nafion-sulfonated polyether ether ketone (SPEEK)-graphene oxide (GO) nanocomposite membranes were prepared. The effects of the degree of oxidation of GO on the physical properties of the nanocomposites have been investigated using two varieties of GOs with low degree (GO) and high degree (HGO) of oxidation. The concentrations of the oxygen functionality in GO and HGO are 284% and 31.8%, respectively as calculated by XPS analysis, but the size of HGO is less than that of GO. Thermal stabilities of the composites are similar except for minor differences in weight loss of various composites due to the presence of different amounts of -SO3H and oxygen functionalities. The storage modulus of the HGO-basecl composite is the highest among the composites and the addition of SPEEK is detrimental to the mechanical properties in all cases. Optimum proton conductivity of 322.2 mS cm(-1) is noted with Nafion containing 1 wt% SPEEK and 0.75 wt% HGO at 100% relative humidity (RH) and 90 degrees C compared to 198 mS cm(-1) under similar conditions. The maximum power density of 621.2 mW cm(-2) has been noted for the same composite membrane at 80 degrees C and 100% RH. (C) 2014 Published by Elsevier B.V.