Temperature-programmed desorption (TPD), temperature-programmed reaction (TPR), temperature-programmed surface reaction (TPSR) and stop flow-TPD techniques as well as FIF-IR spectroscopy of pre-adsorbed NH3 and NO were applied to characterize surface sites present on the titania or alumina pillared montmorillonites modified with Cu or Co cations. For all the catalysts the surface concentration of ammonia was at least twice higher than the amount of chemisorbed NO. Formation of surface NO3- anions was found for all the studied samples, however, thermal stability of nitrates and products of their decomposition depended on a kind of pillars introduced to the clay material. Weakly chemisorbed NO molecules are easy replaced by ammonia under reaction conditions. The reactivity of NH3 and NO pre-adsorbed on the titania pillared montmorillonite modified with copper was significantly higher than that measured for the other studied catalysts. The mechanism of DeNOx process over modified pillared clay (PILC) catalysts depends on the reaction temperature. At the lower temperatures conversion of NO from the gas phase by chemisorbed ammonia dominates, while at higher temperatures also surface NO3- anions can react with chemisorbed NH3 molecules. (C) 2004 Elsevier B.V. All fights reserved.