Molecular interactions between pyridine molecules and titanium dioxide particles of different crystalline modifications (anatase and rutile) have been investigated by IR spectroscopy. Analysis of spectral data allowed to suggest possible mechanisms of these interactions and make some conclusions about TiO2 surface state and active centers. Anatase surface has active centers of both Lewis and Bronsted types (LAC and BAC), with which pyridine interacts via coordination bonding or by the creation of pyridinium-ion (PyH+). Rutile samples have only LAC on their surface, and this does not depend on thermal treatment. Also, pyridine can form hydrogen bonds with hydroxyl groups of TiO2 surface and adsorbed water molecules in these heterogeneous systems. There are at least two types of non-equivalent OH-groups with different bonding enthalpies. Their amount and enthalpy depend on the crystalline modification of TiO2, thermal treatment, and change after loading TiO2 particles into pyridine. (C) 2003 Elsevier Science B.V. All rights reserved.