Fuel-cell dynamics have been investigated with a variable-resistance board applied to a high-temperature polymer fuel cell, operating on pure oxygen and hydrogen at atmospheric pressure and about 150 degrees C. A particular pattern has been identified in the voltage-current density diagram. A model has been developed, using a. characteristic equation to represent the external circuit as a resistive load, possibly nonlinear and time-varying, rather than assuming the current density to be an independent variable. The model successfully explains the shape of the transient current-voltage paths, including the different time constants observed when reversing a transient. (c) 2006 Elsevier B.V. All rights reserved.