Solid oxide fuel cells with thin La(0.9)gSr(0.1)Ga(0.8)Mg(0.1)O(3-delta) (LSGM) electrolytes have the potential to provide high power densities down to at least 600 degrees C. The present study was on a candidate cathode material for such SOFCs: composites containing La0.6Sr0.4Co0.2Fe0.8O3-theta (LSCF) and LSGM. Symmetrical cathodes with LSCF contents of 30-70 wt.% were fired at temperatures from 1000 to 1300 degrees C on both sides of bulk LSGM electrolytes. No secondary phases were detected by X-ray diffraction for the firing temperatures tested. Cathode polarization resistance R-p, measured using impedance spectroscopy, varied little with firing temperature despite substantial changes in cathode microstructure as observed by scanning electron microscopy. R-p varied only slightly with LSCF content, reaching a minimum of similar to 0.18 Ohm.cm(2) at 650 degrees C for 40-60 wt.% LSCF fired at 1100 degrees C. The R-p value was stable over several hundred hours at a current density of 0.5 A/cm(2). The temperature dependence of R-p 20 yielded an activation energy of approximate to 1.7 eV for all compositions. Measurements versus oxygen partial pressure P-O yielded R-p infinity P-O2(0.20). SOFCs with LSCF-LSGM cathodes and thin LSGM electrolytes yielded a maximum power density of 0.57 W/cm(2) at 650 degrees C. (c) 2008 Elsevier B.V. All rights reserved.