Apart from necessary balance-of-plant components, such as pumps, blowers, sensors, and heat exchangers, the fuel processing unit of a high-temperature polymer electrolyte fuel cell (HT-PEFC) system based on autothermal reforming contains three main components: the autothermal reformer, the water-gas shift reactor, and the catalytic burner. In Julich, several generations of these catalytic reactors have been designed, constructed, and manufactured, with a wide range of thermal powers between 13 and 140 kW. Characteristic common features of the respective reactor generations are described as well as their specific structural features. The experimental part of this paper concentrates on investigations with different generations of reactors for autothermal reforming using different diesel and kerosene fuels, which were produced either via the gas-to-liquid or bio-to-liquid process or in a conventional manner from crude oil. They mainly differed from each other with respect to their boiling ranges and mass fractions of aromatics. It was proven in the scope of the current work that autothermal reforming of synthetic kerosene and diesel fuel can be performed for 6000 h of time on stream at a fuel conversion of 99.99 and 99.67%, respectively, The O-2/C molar ratio of the educts was found to be an important reaction parameter strongly affecting the monolith temperatures insider the autothermal reformer and the chemical composition of the product gas. Furthermore, fuel characteristics had a significant impact on reactor performance.