The actual inner workings of a fuel cell are fairly well known, as are the performance maps of the devices composing the balance of plant. It is the interaction of these devices, as controlled by the fuel cell engine's control strategy, which introduces variability to fuel cell engine performance. The control strategy must be able to meet the conditions required for efficient stack operation under all operating conditions associated with a potential application. A Nexa commercial fuel cell engine was selected in order that the findings of this study may be immediately relevant. In the present study, changes in the Nexa's performance are investigated by varying the ambient temperature and relative humidity. Our experiments showed that the ambient temperature directly affects the heat exchanger fan power consumption and the maximum power with a statistically significant effect on net efficiency. The ambient relative humidity played a statistically insignificant role in determining the net efficiency of the Nexa. However, the increased frequency and duration of the anode purges indicate an evidence that flooding was beginning to occur, which suggests that the Nexa's water management control strategy is not aggressive enough to deal with high-temperature and high-humidity conditions. (c) 2008 Elsevier Ltd. All rights reserved.