A novel methodology for the on-line diagnosis of PEMFC, intended to be used in portable control systems, is proposed. First, an equivalent-circuit model for PEMFC is proposed. This model has a low computational cost and it allows reproducing the dynamic behavior of the PEMFC. The relationship among the parameters of this simplified model and PEMFC electrochemical parameters is stated. Secondly, a procedure to estimate all the parameters of this model from the cell's transient response after current interruption (CI) is proposed. The equipment required to make these CI measurements is easily portable and inexpensive. Thirdly, the procedure to calculate some relevant PEMFC electrochemical parameters from the equivalent-circuit parameters is discussed. These PEMFC parameters are the double layer capacitance, the diffusion resistance, the charge transfer resistance, the diffusion-related time constant, and the membrane resistance. Finally, this PEMFC assessment methodology is applied to the diagnosis of the cathode flooding. The diffusion resistance is found to have a dominant effect on the amount of water accumulated inside the cell. The described methodology can be used to characterize other phenomena, including the cathode and membrane drying, the membrane degradation, and the anode poisoning by carbon monoxide. (C) 2007 Elsevier B.V. All rights reserved.