The electrochemical properties of geometrically well-defined Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) microelectrodes have been investigated by impedance spectroscopy. The microelectrodes of 20-100 pm diameter and 100 nm thickness were prepared by pulsed laser deposition (PLD), photolithography and argon ion beam etching. The oxygen reduction reaction at these model electrodes is limited by interfacial processes, i.e. by the oxygen surface exchange and/or by the transfer of oxide ions across the electrode/electrolyte boundary, whereas the resistance associated with the transport of oxide ions through the bulk of the thin film electrode is negligible. The experiments revealed an extremely low absolute value of the electrochemical surface exchange resistance of only 0.09 (+/- 0.03) Omega cm(2) at 750 degrees C in air, which is more than a factor of 50 lower than the corresponding value measured for La0.6Sr0.4Co0.8Fe0.2O3-delta (LSCF) microelectrodes of the same geometry. The dependence of this and other electrochemical quantities such as the chemical bulk capacitance or the BSCF/YSZ interfacial resistance on temperature has been studied between 500 and 750 degrees C. (c) 2006 Elsevier B.V. All rights reserved.