Lanthanum tungstates are a family of promising proton conductors for potential applications in fuel cells and in hydrogen permeation membranes. Single phase 15% and 30% Mo-doped La-27(W0.85Nb0.15)(5)O55.5-delta were synthesized via a solid-state reaction route. The cell parameters of the cubic unit cell with space group F-43m increased from 11.181(7) angstrom to 11.186(5) angstrom when increasing the Mo content from 0% to 30%. The proton concentration in La-27(W0.55Nb0.15Mo0.30)(5)O55.5-delta was similar to La-27(W0.85Nb0.15)(5)O55.5-delta, and the influence of Mo doping on the total, proton and electron conductivity of La-27(W0.85Nb0.15)(5)O55.5-delta was small in wet O-2. The electron conductivity of 30% Mo-doped La-27(W0.85Nb0.15)(5)O55.5-delta in wet 5% H2-Ar increased by more than one order of magnitude compared to the undoped material. The hydrogen flux increased from 4.96 x 10(-5) to 5.82 x 10(-4) ml cm(-1) min(-1) by 30% Mo doping of La-27(W0.85Nb0.15)(5)O55.5-delta with dry Ar as the sweep gas and wet H-2 as the feed gas at 750 degrees C. (C) 2016 The Electrochemical Society. All rights reserved.