Abstract We have studied the interaction of pyridine on oxide surface in order to characterize the Lewis acid sites. Ab initio RHF method was employed at 3-21G* all electron basis set level. The LanL2DZ effective core potential was also used and compared with all electron basis set results. Cluster model of Nb2O5 was built using the crystallographic data. The optimized pyridine molecule was compared to the experimental free molecule showing small variation of ionization energies. The vibrational frequencies of adsorbed pyridine were calculated at RHF/3-21G* level. Population analysis was carried out using Mulliken, ChelpG and NBO schemes. The adsorbed pyridine geometry shows a small deviation of optimized bond angles and interatomic distances relative to free pyridine molecule. The Nb–N interatomic distance is almost 2.41 A.