In order to probe the mechanism of O-2 reduction on mixed-conducting oxides, we have measured the nonlinear harmonic impedance response of a dense thin-film electrode of La0.6Sr0.4CoO3-delta. By combining a well-defined geometry with detection of small nonlinear harmonics, this approach reveals the nonlinear rate law governing O-2 reduction/oxidation. Comparison to various models based on nonequilibrium thermodynamics suggests that O-2 reduction on La0.6Sr0.4CoO3-delta is limited by dissociative adsorption. To the authors' knowledge, this is the first isolation of the full nonlinear rate law governing O-2 exchange on a perovskite oxide. (c) 2007 The Electrochemical Society.