A method for use with an exhaust gas post-treatment system on an internal combustion engine operated with air surplus includes reducing nitrous oxides in that an ammonia-separating reducing agent is added to the exhaust gas flow upstream of a catalyst which is charged with a catalyst material for selective catalytic reduction of nitrous oxides. The method further provides that the NH3 to NOx ratio (feed ratio α) is varied in phases by changing the untreated nitrous oxide emissions of the internal combustion engine such that the feed ratio α oscillates in phases about a predefined value.