A series of Mo/M2O5-TiO2 catalysts with Mo loadings ranging from 1 to 10 wt.% were prepared and characterized by X-ray diffraction (XRD), pulse oxygen chemisorption, laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR). XRD patterns indicate the presence of a crystalline MoO3 phase beyond 7 wt.% Mo on Mb(2)O(5)-TiO2. Molybdena is found to be present in a highly dispersed state at lower Mo loadings. Pulse oxygen chemisorption results reveal that the oxygen uptake is found to increase with Mo loading up to 5 wt.% and levels off at higher loadings. Temperature-programmed reductions of H-2 suggest that reduction of the catalysts occurs in two stages. The reducibility of Mo/M2O5-TiO2 catalysts decreases with increase of Mo loading on the support. Laser Raman spectra shows that the surface molybdate species are present at low Mo loading (<5% Mo) and crystalline MoO3 bands are observed from 7 wt.% of Mo and above loadings. The catalytic activity during ammoxidation of toluene increases with molybdena loading up to 5 wt.%, which corresponds to monolayer coverage and remains unchanged at higher Mo loadings. A linear correlation is observed between catalytic activity and oxygen chemisorption sites. (C) 2004 Elsevier B.V. All rights reserved.