La0.5Sr0.5CoO3 - delta and La0.6Sr0.4CoO3 - delta are characterized with respect to application as cathode materials in intermediate temperature solid oxide fuel cells (IT-SOFCs). Surface exchange and transport parameters of oxygen are determined by the conductivity relaxation technique between 525 degrees C and 725 degrees C at oxygen partial pressures of 0.1, 0.01 and 0.001 bar. Electrical conductivities of both compounds range between 1000 and 2400 S cm(-1) and are slightly higher for La0.6Sr0.4CoO3 - delta. However, La0.5Sr0.5CoO3 - delta shows superior performance with regard to oxygen diffusion, ionic conductivity and oxygen surface exchange within the investigated range of temperatures and oxygen partial pressures. At 725 degrees C the chemical surface exchange coefficient of oxygen is 2 x 10(-3) cm s(-1) for La0.5Sr0.5CoO3 - delta and 4 x 10(-4) cm s(-1) for La0.6Sr0.4CoO3 - delta. At lower oxygen partial pressures a strong decrease in the surface exchange coefficient is observed and diffusion data cannot be obtained from relaxation measurements. Oxygen vacancy diffusion coefficients are similar for both compounds and range between 10(-6) and 10(-8) cm(2) s(-1) with activation energies around 100 kjmol(-1). At 725 degrees C and 0.1 bar oxygen partial pressure, the ionic conductivity of La0.5Sr0.5CoO3 - delta is 1 x 10(-2) S cm(-1) with an activation energy of 118 kjmol(-1). The results show that both compositions meet the requirements for the application as IT-SOFC cathodes in the short-term range. (C) 2012 Elsevier B.V. All rights reserved.