Data are presented to characterize the effects of reservoir size and hydrogen dilution on the dynamic behavior of a proton exchange membrane fuel cell (PEMFC) subjected to rapid changes in the voltage when the flowrates are constant. The data consist of the responses of the current density during low fuel stoichiometries in an effort to expand an understanding of the previously observed overshoot/undershoot behavior. That is, recent studies of the dynamic behavior of a PEMFC have shown pseudo-second-order dynamics of the current response to a change in voltage [J. Power Sources (2004); J. Electrochem. Soc. (2004)]. The data reported here lend further evidence that under fuel starved conditions, rapid changes in the cell voltage between 0.7 and 0.5 V yield pressure differences sufficient to create a "vacuum" effect. This vacuum effect may cause fuel to be drawn from the manifold in a stack or cause ambient air to enter a laboratory scale cell. The vacuum effect explained in our previous work [J. Power Sources (2004)] is shown here to depend on diameter and volume of fuel reservoirs and on the concentration of hydrogen in the fuel. (C) 2004 Elsevier B.V. All rights reserved.