NOVELTY - The selective oxidation-reactor for the purification of carbon monoxide from gas mixture, comprises first- and second tube guiding a cooling agent, a third tube arranged between the first and the second tubes, a fourth tube and a holding device locking the four tubes. The four tubes are coaxially arranged with different radii and are fluidically connected. A temperature measuring point (150) is arranged in the reactor. The holding device has a reactor head and reactor base. The catalyst is a catalyst bed (122). The first tube is an external tube and the second tube is an internal middle tube.    USE - Useful for the purification of carbon monoxide from gas mixture.    ADVANTAGE - The selective oxidation-reactor ensures efficient purification of carbon monoxide from gas mixture due to the compact size of the reactor. The manufacturing cost of the reactor is cheap.    DETAILED DESCRIPTION - The selective oxidation-reactor for the purification of carbon monoxide from gas mixture, comprises first- and second tube guiding a cooling agent, a third tube arranged between the first and the second tubes, a fourth tube and a holding device locking the four tubes. The four tubes are coaxially arranged with different radii and are fluidically connected. A temperature measuring point (150) is arranged in the reactor. The holding device has a reactor head and reactor base. The catalyst is a catalyst bed (122). The first tube is an external tube, the second tube is an internal middle tube, the third tube is an exterior middle tube and the fourth tube is an internal tube of four coaxially arranged tubes. A first supplying line, which supplies gas mixture to the reactor and a second supplying line, which supplies oxygen-containing gas to the reactor are fluidically connected and are arranged to each other in reverse flow direction. The cooling agent is the cooling water of a fuel cell. The third tube comprises a catalyst and is enforceable from the flow of the gas mixture. The reactor head consists of devices for the intake of the supply lines and/or devices for the access of the temperature measuring point and/or devices for the distribution of the gas mixture and/or the cooling agent and/or devices for supplying catalyst into the reactor and/or devices for the distribution of the gas mixture. INDEPENDENT CLAIMS are included for:    (1) a reformer-fuel cell system; and    (2) a method for the purification of carbon monoxide from gas mixture.    DESCRIPTION OF DRAWING(S) - The drawing shows a selective oxidation-reactor for the purification of carbon monoxide from gas mixture.    Oxygen gas supply line (104)    Gas mixture supply line (110)    Reaction chamber (120)    Catalyst bed (122)    Temperature measuring point. (150)