Composite electrodes of La0.8Sr0.2MnO3 (LSM)/La28-xW4+xO54+3x/2 (x = 0.85, "LWO56") on LWO56 electrolytes have been characterized by use of electrochemical impedance spectroscopy vs. pO(2) and temperature from 900 degrees C, where LWO56 is mainly oxide ion conducting, to 450 degrees C, where it is proton conducting in wet atmospheres. The impedance data are analyzed in a model which takes into account the simultaneous flow of oxide ions and protons across electrolyte and electrodes, allowing extraction of activation energies and pre-exponential factors for the partial electrode reactions of protons and oxide ions. One composite electrode was infiltrated with Pt nanoparticles with average diameter of 5 nm, lowering the overall electrode polarization resistance (R-p) at 650 degrees C from 260 to 40 Omega cm(2). The Pt-infiltrated electrode appears to be rate limited by surface reactions with activation energy of similar to 90 kJ mol(-1) in the low temperature proton transport regime and similar to 150 kJ mol(-1) in the high temperature oxide ion transport regime. The charge transfer reaction, which makes a minor contribution to R-p, exhibits activation energies of similar to 85 kJ mol(-1) for both oxide ion and proton charge transfer. (C) 2016 The Electrochemical Society. All rights reserved.