A series of titania (anatase)-supported vanadia catalysts ranging in V2O5 content from 0.4 to 9.9 mol% was prepared by wet impregnation technique, characterized by BET surface area measurement and X-ray diffraction, and evaluated for ammoxidation of 3-picoline. The average oxidation number of vanadium in the fresh and used catalysts was determined by titrimetric methods. The ammoxidation activity and the average oxidation number were observed to increase with vanadia loading up to 3.4 mol% in the catalyst which corresponds to a monolayer coverage. The phase transformation of anatase to rutile after the reaction was observed at a V2O5 loading of 5.9 mol%. The slow decrease of ammoxidation activity beyond 3.4 mol% V2O5 was attributed to the coverage of active monomeric VOx species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia.