The dynamic response of the stirred tank reactor (STR) polymer electrolyte membrane (PEM) fuel cell has been explored over the temperature range of 35-105 degreesC. When the fuel cell was operated in the autohumidification mode the fuel cell current "ignited" when the membrane water content was greater than a critical level of similar to 1.6 H2O/SO3, and it extinguished when the initial membrane water content was below this critical level. Above 70 degreesC, two stable "ignited" states were observed at inter-mediate load resistances; these steady states corresponded to different levels of membrane hydration. At low load resistances only a single ignited steady state was observed with high membrane hydration, and at high load resistances only a single ignited steady state was observed with intermediate membrane hydration. Hysteresis between the two ignited states was observed; the steady state selected depended on the initial conditions in the fuel cell. The time constant for the fuel cell current to reach steady state after a change in the load resistance was similar to 10(3) - 10(4) s. Below 70 degreesC only one "ignited" state and the extinguished state were observed in the autohumidification fuel cell. After 3000 h of operation the STR PEM fuel cell current and effluent relative humidities oscillated autonomously between two membrane hydration states with a period of oscillation of similar to 10,000 s. The oscillations showed abrupt transitions indicative of a capacitive switch. These complex dynamics of PEM fuel cell operation are associated with the membrane water uptake. It is hypothesized that water produced and swells the membrane, altering the interfacial membrane-electrode contact. (C) 2004 Elsevier Ltd. All rights reserved.