The adsorption of N/sub 2/0 in the temperature range 150-650 degrees K was investigated over model active and deactivated de-NO-ing catalysts. Powders having the compositions TiO/sub 2/, V/TiO/sub 2/, W/TiO/sub 2/, K-V/TiO/sub 2/ and K-W/TiO/sub 2/ were taken into account. Molecular adsorption by coordination over surface cations through both the N- and the 0- end of the N/sub 2/O molecule was observed in all cases. The vibrational perturbation depends on the Lewis acid strength of the surface cations. These species are reversible at r.t. by short evacuation. Pre-adsorption of water and ammonia poisons the adsorption sites for N/sub 2/O, due to the much stronger basicity of these molecules. Evidence of the reaction of N/sub 2/O with adsorbed ammonia on V/TiO/sub 2/ is found only at 623 degrees K, unlike that for NO which oxidizes NH/sub 3/ starting already from 423 degrees K.