Previously unrecognized amorphous grain boundary films are shown to provide rapid proton conduction paths in polycrystalline lanthanum phosphates. Experiments reveal the amorphous phosphorus-rich grain boundary films to be stable in air at 773 K, with a conductivity upwards of 2.5 x 10(-3) S/cm at 773 K, many orders of magnitude higher than in the crystalline bulk. The results indicate the possibility of forming stable, high proton-conductivity nanocomposites of RE-phosphate glasses and crystalline ceramics for intermediate temperature (573 K to 773 K) fuel cells. Published by Elsevier B.V.