A novel diagnosis methodology for characterizing the cathode flooding, the membrane drying and the anode catalyst poisoning by carbon monoxide is proposed in this manuscript. This diagnosis methodology is intended to be used in portable control systems. Two equivalent-circuit models for PEM fuel cells (PEMFC) with a low computational cost are proposed. The first model describes the dynamic behavior of the PEMFC during its normal operation, and also during the cathode flooding and the membrane drying processes. A method to estimate the parameters of this model is proposed. The experimental procedure for data acquisition does not interfere significatively with the cell operation, and the measurement equipment is easily portable and inexpensive. The second model reproduces the cell dynamic behavior when the anode catalyst is poisoned by carbon monoxide. The relationship among the parameters of these equivalent-circuit models and PEMFC electrochemical parameters is stated. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.