Composite membranes composed of zirconium phosphate (ZrP, a proton conductor), and porous polytetrafluoroethylene (PTFE, a mechanical support for ZrP), have been studied as electrolytes for direct hydrocarbon fuel cells that might operate at temperatures approaching 200 degrees C. The previous literature describes membranes formed by compressing PTFE particles and ZrP particles (conductivity = 10(-3) S cm(-1)). The results reported here show that adding glycerol (GLY) to a reaction mixture of ZrOCl(2).8H(2)O and H(3) PO(4) Pay to form ZrP in situ within the pores of PTFE, produced a membrane (conductivity = 0.02-0.045 S cm(-1)) that approached the performance of Nation (conductivity = 0.1 S cm(-1)). The conductivity remained unchanged when one of the membranes (conductivity = 0.02 S cm(-1)) was processed at the inlet conditions to a direct propane fuel cell (200 C and steam mole fraction yH(2)O = 0.86). The composite membrane, prepared with glycerol, contained ZEP spheres (100-500 nm) that were smaller than the PTFE pore diameters (1000-2000 nm). The enhanced conductivity may have been caused by a combination of proton transport on the exterior surfaces of the ZrP solid spheres, proton hopping through the bulk of the ZrP, and proton hopping via the OH groups in glycerol. (C) 2011 Elsevier B.V. All rights reserved.