Composite membranes based on poly(2,2'(m-phenylene)-5,5'bibenzimidazole) (PBI) and sulfonated polyhedral oligosilsesquioxane (S-POSS) with S-PUSS contents of 5 and 10 wt.% were prepared by solution casting as base materials for high temperature polymer electrolyte membrane fuel cells. With membranes based on pure FBI as a reference point, the composite membranes were characterized with respect to spectroscopic and physicochemical properties. After doping with phosphoric acid, the composite membranes showed considerably improved ex situ proton conductivity under anhydrous as well as under fully humidified conditions in the 120-180 degrees C temperature range. The conductivity improvements were also confirmed by in situ fuel cell tests at 160 degrees C and further supported by the electrochemical impedance spectroscopy data based on the operating membrane electrode assemblies, demonstrating the technical feasibility of the novel electrolyte materials. (C) 2014 Elsevier Ltd. All rights reserved.