A method and an apparatus for posttreatment of the exhaust gases of an internal combustion engines in which nitric oxides contained in the exhaust gas can be selectively catalytically reduced, and a first auxiliary agent kept on hand can be delivered to the exhaust gas, and means are provided for at least intermittently subjecting a portion of the first auxiliary agent to a chemical conversion into a second auxiliary agent, and an intermediate reservoir is provided for storing the second auxiliary agent, so that at least intermittently, the second auxiliary agent can be delivered to the exhaust gas parallel to or in alternation with the first auxiliary agent.