A new reactor concept for the autothermal operation of endothermic high-temperature reactions is presented. An endothermic synthesis reaction is coupled with a combustion reaction in such a way that both reactions take place in adjacent channels of a countercurrent fixed-bed reactor. Due to the countercurrent heat exchange the feed and the exit of both reactions have low temperatures while a high-temperature zone in the middle of the reactor allows for high conversion with a subsequent rapid quench. The applicability of the concept is shown through detailed simulation studies and first experiments. As an example reaction methane steam reforming is considered. A ceramic honeycomb monolith has been used as countercurrent heat exchanger where the catalyst was deposited on the monolith walls. First experimental results show the general applicability of the concept. Due to the limited thermal stress resistance of the ceramic monolith used the maximum operation temperature has so far been limited to 800 degrees C.