Co is doped to Sr2Fe1.5Mo0.5O6 to enhance its electrochemical activity as the cathode for intermediate-temperature solid oxide fuel cells. Pure cubic perovsldtes of Sr(2)Fet(1.5-x)Co(x)Mo(0.5)O(6) (SF1.5-xCxM, x = 0, 0.5, 1) are synthesized using a glycine-nitrate combustion progress. The average thermal expansion coefficient varies from 15.8 to 19.8 x 10(-6)K(-1). The electrical conductivity increases while its activation energy decreases with increasing Co content. X-ray photoelectron spectroscopy analysis demonstrates mixed valences of Fe, Co and Mo, suggesting small polaron hopping mechanism. Electrical conductivity relaxation (ECR) measurement shows that the surface exchange coefficient increases about two orders of magnitude when the content increases from x = 0 to x = 1.0, i.e. from 2.55 x 10(-5) to 2.20 x 10(-3) cm s(-1) at 750 degrees C. ECR also exhibits that chemical diffusion coefficient increases with Co content. Density Functional Theory calculation demonstrates that oxygen vacancy formation energy decreases with Co content, suggesting high oxygen vacancy concentration at high Co content. Impedance spectroscopy on symmetric cells consisting of SF1.5-xCxM electrodes and La(0.8)Sr(0.2)Oa(0.3)Mg(0.2)O(3-delta) electrolytes shows that Co doping is very effective in reducing the interfacial polarization resistance, from 0.105 Omega cm(2) to 0.056 Omega cm(2) at 750 degrees C. These results suggest that Co doping into Sr2Fe1.5Mo0.5O6 can substantially improve its electrochemical performance. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.