The invention relates to a procedure for controlling the reducing agent in an exhaust gas post-processing system of a combustion machine with an exhaust gas duct, in which an SCR catalytic converter is provided in the direction of flow of the exhaust, wherein a reducing agent generation system has an NOx and CO/H2 generation unit,, an oxidation catalytic converter and a combined NOx storage / ammonia generation unit in the standard gas route of the reducing agent generation system, and, in order to reduce nitrogen oxide, ammonia is added as a reducing agent in front of the SCR catalytic converter by the reducing agent generation system, wherein source materials for the generation of ammonia are at least intermittently added to the NOx and CO/H2 generation unit via a fuel supply line and via an air supply line, wherein during the fat phase the air/fuel composition is changed in front of the oxidation catalytic converter in the form of a lambda modulation for a lambda value. It can thus be achieved that, in pulse mode, a high H2/CO yield can be achieved with a simultaneously low HC slip and the higher HC penetrations associated with the natural aging of the catalytic converter and the decreasing H2/CO yields are compensated. Influences resulting from the reactor geometry of the oxidation catalytic converter can be influences or compensated with the procedure in terms of its drive-away characteristic.