The PEM fuel cell operating conditions reported here are unusual; almost all our results are at low relative humidity in the cell, most of them to relative humidity of < 40%. We are not advocating operating a commercial PEM fuel cell in the mode or configuration we employed in our experiments.However, it is essential to know how a PEM fuel cell operates in all conditions to understand how to effectively control it under extremes of operation. Operation of a PEM fuel cell under low humidification conditions accentuated the effects of membrane swelling and highlighted the impact of membrane mechanical properties. Researchers at GM have recently been suggesting the desirability to operate PEM fuel cells with under-humidified feed streams (Mathias et al., 2004). The results reported here suggest there are many complexities with such operation, beyond the reduced proton conductivity.