The influence of vanadium loading and calcination temperature on the catalytic performance of vanadia/TiO2 (mixed-crystal) catalysts for the selective hydroxylation of benzene was investigated. A series of VO (x) /TiO2 catalysts were prepared using a range of vanadium loadings (2.27-9.06 wt %) and calcination temperatures of 450-650 A degrees C. The samples were characterized using thermogravimetry-differential thermal analysis, N-2-adsorption, scanning electron microscopy, H-2 temperature-programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was found that vanadium exists as monomeric and polymeric VO (x) and V2O5 crystal phases, depending on the amount of vanadium, accompanied by transformation of the TiO2 carrier from anatase to rutile. The influence of temperature on the anatase to rutile transformation was strong, and only a little anatase was transformed to rutile at temperatures below 550 A degrees C. When the temperature was raised to 650 A degrees C, rutile became the main crystal phase. Monodisperse vanadia/TiO2 (mainly anatase) catalysts are highly active in benzene hydroxylation to phenol, but aggregation of VO (x) and crystalline V2O5 supported on a rutile carrier lowers the catalytic activity. In addition, the catalytic performances of the various catalysts in hydroxylation of benzene to phenol were investigated and possible reaction mechanisms are discussed.