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 °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.