Effect of H2O and SO2 on the NO reduction activity of the Pd/titania catalysts was examined in the NO+CH4+O-2 reaction. Steady-state and transient response experiments were performed using a tubular flow reactor. Isotopically labeled methane, (CH4)-C-13, was used in studying the transient behavior of the catalytic system. Reactor effluents were analyzed by using gas chromatography, mass spectrometry, and chemiluminescense techniques. Steady-state reaction experiments showed that the catalyst has a high activity for reduction of NO over a wide oxygen concentration range (0-9800 ppm at 500 degrees C). Higher N-2 selectivities (less NH3) and higher methane conversion values were obtained at higher O-2 concentrations. Experiments also showed that NO conversion was not affected by the presence of water (up to 6%), and higher nitrogen selectivities were obtained at higher H2O concentrations in the feed. On the other hand, significant and irreversible loss of activity was observed in the presence of SO2. The reaction experiments indicated the formation of SO3 and H2S over the catalyst. The post-reaction XPS analysis of the catalyst samples showed that sulfate species were formed on the surface and that the surface palladium was completely converted to pd(2+) State after being exposed to SO2.