Potential-dependent Fourier transform infrared emission spectroscopy (pd-FTIRES) has been used for the first time to study, in situ, oxygen reduction under practical operating conditions of intermediate-temperature solid oxide fuel cells. Surface emission difference spectra of functioning cathodes at 550degrees C under a variety of feed gas conditions and overpotentials were obtained. Various surface oxygen anions were observed on the cathode surface under specific oxygen concentrations. The interfacial structural data were also correlated to ac impedance and chronoamperometric measurements. In addition to the bands associated with adsorbed oxygen anions, broad spectral features arising from the electrochemical polarization were also observed. These spectral features are believed to be due to electrochemically induced changes in the optical properties of the emitting surface layer.