PEFC cannot be operated for extended periods because it has some problems. However, if the deterioration factors can identified on time, PEFC can be operated for extended periods by overcoming these problems. Therefore, performance diagnostics needs to be developed to judge the PEFC deterioration factors instantaneously and simply. MCFC performance diagnostic has now been developed by the current-pulse method. Therefore, a thesis was examined relating to the applicability of the diagnostic to a PEFC characteristic diagnosis. The diagnostic evaluates changes in the seven parameters of the fitting equation led from PEFC's transient response between 100 ms obtained by the current-pulse method. The relationship between these parameters and each deterioration factor is evaluated using a PEFC single cell whose electrode area is 25 cm(2). In this case, deterioration factors include cell temperature, the flooding phenomenon and the gas composition/flow rate. As a result, each of seven parameters in the transient response of PEFC corresponds to an ohmic potential drop, anode/cathode gas diffusion resistance, reactive resistance, three-phase interfacial resistance and electrolyte properties, respectively. When the flooding phenomenon is caused in the cathode channel, the transient response of the PEFC obviously differs from the standard value. The change in each parameter is examined by gradually raising the humidity of the cathode supply gas (flooding gradually being caused in the cathode channel). The influence of the flooding phenomenon clearly appeared in three parameters, and in the same way, the temperature dependency of each parameter is confirmed. Therefore, the proposed performance diagnostic will be able to specify the PEFC deterioration factors while driven, easily and on time. (c) 2006 Elsevier B.V. All rights reserved.