The polarization resistance (R-p) of solid oxide fuel cell (SOFC) cathodes is inversely proportional to the square root of Po-2 or R-p infinity Po-2(-1/2). This Po-2 dependence has usually been explained as being due to the slow diffusion of oxygen adatoms. In the transient-state analysis approach, the diffusion limitation hypothesis predicts that the rate of the polarization relaxation characterized by the time constant (t(0)) of the decay curves is independent of Po-2. This prediction is also applicable to the relationship between the impedance spectroscopic behavior characterized by the mathematically equivalent omega(max) (=1/t(0)) and Po-2, where omega(max) is the frequency at the top of a corresponding semicircle in the Cole-Cole plots. In fact, however, t(0) decreases and omega(max) increases with increasing Po-2. This study attempted to resolve this disagreement based on an inhomogeneous electrode model, which assumed that the electrode is inhomogeneous in terms of the local polarization resistance and therefore the polarization is locally inhomogeneous. The inhomogeneous electrode model attributed the polarization relaxation to the interfacial current, which passes through the interfacial short circuits formed between the differently polarized local areas. As Po-2 increased, the potential difference between the local areas became greater. Thus, the polarization relaxed in a shorter time, resulting in a smaller t(0) value. The experimental observation that t(0) is more dependent on Po-2 as the electrode becomes more inhomogeneous provided a strong argument in favor of the inhomogeneous electrode model. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3545001]