Balance-of-plant components, especially the air supply system, have a critical impact on the operating condition, performance, and reliability of polymer electrolyte fuel cells (PEMFCs). Thus, we investigated the performance and operating characteristics of a coupled system integrating a PEMFC and an air supply unit. The performance characteristics of the fuel cell stack were modeled using a semi-experimental correlation, and the operating characteristics of a shell-and-tube type membrane humidifier was modeled using heat and mass transfer principles. The models of both components were validated. A turbo-blower determined the condition of the air supplied to the humidifier and its characteristics were modeled using a performance map. A program was developed to simulate the operation of a PEMFC system consisting of the fuel cell stack and the air supply unit, and its operating characteristics at various conditions were investigated. In particular, the effects of operating conditions (ambient temperature and load) on the performance of both the humidifier and the fuel cell stack were examined, and the variations of critical operating parameters were analyzed.