It is generally accepted that the ideal operating temperature of a molten carbonate fuel cell (MCFC) is 650 degreesC. Nevertheless, when waste heat utilization in the form of an expander and steam production cycle is introduced in the system, another temperature level might prove more productive. This article is a first attempt to the optimization of MCFC operating temperatures of a MCFC system by presenting a case study in which the efficiency of a combined beat and power (CHP) plant is analyzed. The fuel cell plant under investigation is designed around a 250kW-class MCFC fuelled by natural gas, which is externally reformed by a heat exchange reformer (HER). The operating temperature of the MCFC is varied over a temperature range between 600 and 700 degreesC while keeping the rest of the system the same as far as possible. Changes in energetic efficiency are given and the causes of these changes are further analyzed. Furthermore, the exergetic efficiencies of the system and the distribution of exergy losses in the system are given. Flowsbeet calculations show that there is little dependency on the temperature in the first order. Both the net electrical performance and the overall exergetic performance show a maximum at approximately 675degreesC, with an electrical efficiency of 51.9% (LHV), and an exergy efficiency of 58.7%. The overall thermal efficiency of this CHP plant increases from 87.1% at 600 degreesC to 88.9% at 700 degreesC. Overall, the change in performance is small in this typical range of MCFC operating temperature. (C) 2003 Elsevier Science B.V. All rights reserved.