Cation diffusion in functional oxides exposed to electrochemical gradients may lead to kinetic demixing or decomposition and, consequently, determine the life-time of the functional component. Here we present chemical diffusion coefficients of Nd and Mo in the mixed proton-electron conductor lanthanum tungsten oxide, La28 - xW4+ xO54+ 3x/2 (LWO), measured at 1000 to 1200 degrees C in both oxidizing and reducing atmospheres. The bulk diffusivities of Nd and Mo were similar at all temperatures investigated and did not change significantly from oxidizing to reducing conditions. On these bases it is suggested that bulk diffusion of both Nd and Mo occurs via the La2 site on which both cations reside. Based on the low activation energy for bulk transport (similar to 200 kJ.mol(-1)) at temperatures below 1200 degrees C it is proposed that the cation defect concentrations are, in effect, frozen in. Preferential diffusion of Nd along the grain boundaries was rationalized based on space charge effects and depletion of W6+ and MO6+ near the positively charged grain boundary core. Potential implications of kinetic demixing or decomposition of EWO membranes are also evaluated based on the present results. (C) 2015 Elsevier B.V. All rights reserved.