La0.5Sr0.5MnO3 thin films were prepared by a pyrosol technique. Films were characterized by x-ray diffraction and scanning electron microscopy analysis. Their electrical resistance was measured, and they showed a semiconductive behavior. The eta(i) curves were drawn at 800 and 900-degrees-C and impedance spectroscopy was performed during polarization. At low polarization, the oxygen reduction takes place at the triple contact points and is progressively delocalized over the whole surface of the electrode. At high polarizations, two components of the impedance have been identified. The first, at high frequency, is correlated with an atomic rearrangement of the electrode (either in the bulk or at the surface), which favors the electrode reaction. At low frequencies, the impedance has been attributed to a concentration impedance due to at least two adsorbed species.