The oxygen reduction (OR) mechanism at the Sr-doped (LSM) and yttria stabilized zirconia (YSZ) composite cathode for high temperature solid oxide fuel cells is still uncertain, despite of the great deal of work carried out over the last years about this system. In previous works, we tested a half-cell (with a YSZ electrolyte pellet) in a typical three-electrode configuration: It was observed that the portion of the composite cathode volume involved in the reaction depends on the operating temperature. Moreover, we analyzed part of the impedance data by the differential impedance analysis, which does not need a preliminary working hypothesis. The results suggested that significant limitations in the oxygen ion transport occur in the LSM pure material, which are not observed in the composite YSZ/LSM cathode. In this study, we investigate the behavior of the LSM/YSZ system in a Ni/YSZ cermet anode-supported half-cell with yttria stabilized zirconia (8YSZ) electrolyte and a screen printed LSM/YSZ composite cathode. The aim is to individuate and characterize the cathodic contribution from the overall impedance response, varying the partial pressure of the reactant gases, to obtain additional information about the OR mechanism from the dependence. By a possible interpretation of the oxygen reaction mechanism, a comparative study of the cathode behavior with previous results is performed.