Effects of co-existence of H2O, CO2, and SO2 in the reaction gas on the catalytic performance of cobalt containing silicate having ZSM-5 (MFI) structure (Co-silicate) were studied. Water vapor retarded only the hydrocarbon conversion to CO2 but no effect was observed on NO conversion. Addition of CO2 or SO2 did not affect the reaction. The stability of H-Co-silicate against H2O, CO2, and SO2 was ascribed to the state of metallic active species which were stabilized by incorporation into the high siliceous MFI structure.