The voltage dynamic properties of PEM fuel cell stack have been analyzed through experimental investigation. Different behaviours between voltage overshoot and undershoot are found under load commutations. A semi-empirical dynamic model for stack voltage is introduced on the basis of experimental investigation. The proposed model can predict the transient response of stack voltage under step change in current. Comparing with previous model, the suggested model indicates a better agreement between tests and simulations. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.