Oxides of La0.8Sr0.2Mn1-xRuxO3-delta (LSMR) (x = 0, 0.25, 0.50, 0.75, or 1.0) were prepared to fabricate cathodes in solid oxide fuel cells. The crystal structure changed from trigonal (x = 0 or 0.25) to a mixture of trigonal and orthorhombic (x = 0.5) and to orthorhombic (x = 0.75 or 1.0). X-ray photoelectron spectroscopy analysis after electrochemical testing indicated that the relative concentrations of Ru4+ to Ru6+ and Mn4+ to Mn2+ influence the performance of a single cell. The transformation from Ru4+ to Ru6+ releases two electrons but that from Mn4+ to Mn2+ creates two electron holes (an oxygen vacancy). The relative concentrations in LSMR were determined using the stoichiometric ratio (x) of Ru, and then, the concentrations of electrons and electron holes for influencing the cathode electrochemical catalytic reactivity were estimated. x = 0.25 represented the better cell performance, and Ru may stabilize the LSMR grain size during electrochemical testing. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.