An exhaust gas post treatment system for nitrogen oxide and particle reduction of internal combustion engines operated with excess air. An oxidation catalytic converter is disposed in the exhaust gas stream of the engine for converting at least a portion of the nitric oxide in the exhaust gas into nitrogen dioxide. The first particle separator or filter is disposed in the exhaust gas stream downstream of the oxidation catalytic converter for converting carbon particles accumulated in the separator or filter into carbon monoxide, carbon dioxide, nitrogen and nitric oxide with the aid of nitrogen dioxide contained in the exhaust gas. A partial exhaust gas stream is branched off from the exhaust gas stream upstream of the first separator or filter. A metering device adds reduction agent to the partial exhaust gas stream in the form of ammonia or a material that releases ammonia downstream of the supply location due to hot exhaust gas. A second particle separator or filter is disposed in the partial exhaust gas stream downstream of the supply location. The partial exhaust gas stream returns to the exhaust gas stream downstream of both particle separators or filters. An SCR catalytic converter is disposed downstream of the return location for reducing nitrogen oxides in the exhaust gas to nitrogen and water vapor with the aid of ammonia or released ammonia by way of selective catalytic reduction.