Supported by the Ministry for Environment, Energy and Federal Affairs (HMUEB) of the German State of Hesse, the local electricity supplier Hessische Elektrizitats-AG HEAG demonstrated a 200 kW PAFC cogeneration plant of type ONSI PC25 at Darmstadt, Germany. The demonstration was accompanied by an experimental program dealing with the transient and long-term behaviour of the unit and investigating its gas process technology. The relevant cell parameters such as cell voltage, current, cell and system efficiency have been monitored continuously during almost 20 000 h of hot time. The concentrations of carbon monoxide, hydrocarbons and nitrogen oxides of the stack gas amount to less than 6.5, 5.5 and 1.5 ppm. The product water released from the plant is free of heavy metals and other contaminants. Still, ageing seems to proceed too fast and cell and maintenance costs are too high, resulting in electric energy costs of 0.28 DM/kWh. The next generation of this fuel cell type, however, may well be able to compete with diesel cogeneration plants.