A catalytic after treatment system for lean HC-SCR was constructed of two different catalyst beds, e.g. of a Ag/alumina and Cu-ZSM-5 catalyst (cascade concept). The improved activity especially at low temperature range was found to be due to the synergetic effect of the two catalysts, which combines the transformation of the feed gas over Ag/alumina to such compounds that are highly reactive towards N-2 over Cu-ZSM-5. The effluent coming from the Ag/alumina bed was analysed by GC-MS along with the NO to N-2 conversion over the whole system by GC. The results obtained from the GC-MS measurements revealed that hydrocarbon used as a reducing agent is oxidised and that besides oxygenates also various N-containing hydrocarbons are formed over the Ag/Al2O3.