Cyclic voltammetry and steady state polarization measurements were performed for the electrochemical characterization of the O-2(g), Pd/YSZ system. The effect of oxygen partial pressure was investigated in the temperature range of 350-400 A degrees C and it was found that under polarization phase transformations can take place. Application of negative potential values, lower than a threshold value of -0.6 V, can result in the decomposition of PdO, while the reduced species can be re-oxidized during a linear anodic potential scan. The results suggested that during the first minutes of cathodic polarization, the PdO reduction reaction dominated versus the oxygen reduction reaction, while it becomes more significant as the oxygen concentration in the gas phase increased within the range 0-6 kPa.