Dynamic responses of PEM fuel cells are crucial for mobile applications such as in automobiles. There are four main transient processes in a PEM fuel cell, namely, species transport, electric double layer charge/discharge, membrane hydration/dehydration, and heat transfer. In this study, a rigorous transient model has been developed, accounting for all four transient mechanisms. The dynamic characteristics have been analyzed, corresponding to various changes in working conditions, such as relative humidity and/or cell voltage. Moreover, by using three different membrane types, Nafion 112, Nafion 115, and Nafion 117, the effect of membrane thickness on the cell dynamic performance has been investigated, and the importance of heat transfer effects on the cell dynamic responses have been highlighted. (c) 2006 Elsevier B.V. All rights reserved.