The device is a component of a system for posttreatment of the exhaust gas of a self-igniting internal combustion engine; by means of the mixture of reducing agent and air that it generates, the action of a reduction catalytic converter is improved. The device has a mixing chamber (22), into which reducing agent, via a reducing agent supply means controlled by a metering valve (24), and air, via an air supply means, can be introduced. The mixing chamber (22) is embodied in a mixing chamber body (20), with which a valve body (26) of the metering valve (24) is connected. The mixing chamber (22) discharges, toward the metering valve (24), into a connection stub (42), and the metering valve (24) has a valve member (74), which to control the reducing-agent/air mixture supply means into the mixing chamber (22) cooperates at least indirectly with the face end (43) of the connection stub (42) as a valve seat. The valve body (26) is screwed into a bore (38) of the mixing chamber body (20). The result is a simpler design of the device, with only a few components which are simple to assemble.