The selective catalytic reduction (SCR) of NOx by dodecane in excess oxygen-containing gas mixtures was studied on Ce and/or Pd-loaded HMOR. Under dry conditions the best fresh bimetallic catalyst, 6.2 wt.% Ce-0.08 wt.% Pd-HMOR, displayed a maximum of about 70% NOx conversion to N-2 at 350 degreesC and GHSV = 30,000 h(-1), while either Ce-HMOR or Pd-HMOR exhibited low activity for the SCR reaction. The presence of both Ce and Pd in the zeolite was crucial for high deNO(x) activity. The dodecane concentration is very important for the reaction. The catalyst is not active in the absence of the reducing agent and inhibition of NOx reduction is observed at dodecane concentrations higher than 440-500 ppm. The presence of 40 ppm SO2 in the gas feed suppresses the reaction. However, the coexistence of 15% H2O and 40 ppm SO2 has no appreciable effect on the catalyst activity. Enhanced activity and a broader temperature window was observed in the NO2 + dodecane + O-2 reaction in the presence of 15% H2O and 100 ppm SO2. In this reaction mixture, the catalyst was capable of retaining a stable NO2 conversion to N-2 for a period of ten days. Characterization by XPS and UV-VIS-diffuse reflectance spectra (UV-VIS-DRS) of selected fresh and aged catalyst samples indicates that Pd exists mainly as Pd2+ cations in Ce-Pd-HMOR. Cerium is mostly present as CeO2 on the surface of the zeolite particles. However, part of the cerium in 6Ce-Pd-HMOR, exists as stable Ce3+ species.