Fresh and industrially spent VTiSbSiOx ammoxidation catalysts were used for nicotinonitrile manufacture from 3-picoline in various catalytic tests. The solid-state characterization of these materials was carried out using different techniques. Catalytic runs with spent samples showed a significant drop in catalytic activity in comparison to the fresh catalyst. However, neither coke-containing deposits nor loss of catalyst components could be detected in the spent samples. X-ray diffraction, nitrogen adsorption, UV/VIS- and EPR-spectroscopy were applied to uncover structural alterations during industrial long-term use and to explore possible reasons for the observed deactivation behavior. Characterization by UV/VIS revealed a partial reduction of vanadium (V) and (IV) towards Vanadium(III). EPR showed structural changes leading to a more pronounced formation of antiferromagnetically interacting vanadium (IV) oxide clusters and, thus, to a loss in vanadium dispersion. A slight increase in crystallinity of the spent samples could be observed by XRD. All these effects collectively lead to the observed deactivation; however, the original activity can be restored by re-oxidation under airflow at 600 degrees C. By means of regeneration, reduced vanadium species (mainly V(111)) can be partly re-oxidized and the dispersion of the vanadium (IV) oxide clusters can be enhanced again as evidenced by EPR. Interestingly, the catalytic properties of regenerated samples are comparable to the fresh solid. (c) 2007 Elsevier B.V. All rights reserved.