Amorphous NbMCM-41 and crystalline NbY zeolite were characterized by the adsorption of NO (or oxygen admission followed by NO adsorption) combined with FTIR measurements. XRD, XPS and UV-vis results proved that in both types of materials Nb was present as +5 cation in a tetrahedral coordination involved in Nb-O-Si bonds and no extra framework niobium species were formed. Despite these common features, a substantial difference between the niobium species in these two types of materials was indicated by NO adsorption and FTIR study. In NbMCM-41 activated at 673 K, unsaturated Nb delta+ formed Nb-NO complexes whereas NbO delta- species were responsible for nitrate formation. In NbY zeolite the niobium cations in the framework were saturated and did not form Nb-NO complexes. The interaction of NO with the residual water was documented by the formation of N2O after NO adsorption on NbY. The increase in basicity of Y zeolite after Nb introduction was proved by the formation of nitrite species resulting from electron transfer from the zeolite to NO2. (c) 2011 Elsevier B.V. All rights reserved.