Two PEMFC failure modes (dehydration and flooding) were investigated using in situ electrochemical impedance spectroscopy (EIS) on a four-cell stack under load. The EIS measurements were made at different temperatures (70 and 80 degrees C), covering the cur-rent density range 0.1-1.0 A cm(-2), and the frequency range 0.1-2 x 10(5) Hz. Dehydration and flooding effects were observed in the frequency ranges 0.5-10(5) and 0.5-10(2) Hz, respectively. We propose that impedance measurements at separate frequency ranges (or narrow bands thereof) can be used to distinguish between flooding and dehydration events. Similar approaches may be used to diagnose other important PEMFC failures (c) 2006 Elsevier B.V. All rights reserved.