A composite membrane based on a non-stoichiometric composition of BPO4 with excess of PO4 (BPOx) was synthesised and characterised for medium temperature fuel cell use (120-180 degrees C). The electrolyte was characterised by FTIR, SS-NMR, TGA and XRD and showed that the B-O is tetrahedral, in agreement with reports in the literature that boron phosphorus oxide compounds at B:P < 1 are exclusively built of borate and phosphate tetrahedra. Platinum micro electrodes were used to study the electrolyte compatibility and stability towards oxygen reduction at 150 degrees C and to obtain kinetic and mass transport parameters. The conductivities of the pure BPOx membrane electrolyte and a Polybenzimidazole (PBI)-4BPO(x) composite membrane were 7.9 x 10(-2) S cm(-1) and 4.5 x 10(-2) S cm(-1) respectively at 150 degrees C, 5%RH. Fuel cell tests showed a significant enhancement in performance of BPOx over that of typical 5.6H(3)PO(4)-PBI membrane electrolyte. The enhancement is due to the improved ionic conductivity (3x), a higher exchange current density of the oxygen reduction (30x) and a lower membrane gas permeability (10 x). Fuel cell current densities at 0.6 V were 706 and 425 mA cm(-2) for BPOx and 5.6H(3)PO(4)-PBI, respectively, at 150 degrees C with O-2 (atm). (C) 2015 The Authors. Published by Elsevier B.V.