Electromotive force (EMF) measurements are often used to estimate the transport number of conducting species in ionic and mixed conductors. In this work, the applicability of the method to systems, where both protonic, oxide-ionic and electronic conduction is significant, was examined. The effect of electrode polarisation resistance, water vapour concentration and impurities was analysed. Samples of StCe(0.95)Y(0.05)O(3-delta) were prepared and used in a number of EMF measurements, including wet and dry hydrogen and oxygen concentration cells. It was found that concentration cells including water vapour gave large errors in the determination of the oxide-ionic transport number, but gave good estimates of the protonic transport number. The protonic transport number was estimated to around 0.80 at 600 degrees C, decreasing to around 0.65 at 800 degrees C, consistent with previous investigations on similar materials. (C) 2009 Elsevier B.V. All rights reserved.