A series of MoO3 catalysts with Mo loadings, ranging from 2-12% w/w, supported on TiO2 (anatase) and TiO2 (rutile) were investigated by X-ray diffraction, temperature programmed reduction (TPR). H-1 magic angle spinning (MAS) NMR aisi oxygen chemisorption measurements. Dispersion of molybdena was determined by the oxygen chemisorption at 623 K and by a static method on the samples prereduced at the same temperature. At low Mo loadings, i.e. below 6% Mo. molybdenum oxide was found to be present in a highly dispersed state. Oxygen chemisorption results suggest that MoO3 particles disperse better on TiO2 (anatase) than when they are supported on TiO2 (rutile). H-1 magic-angle-spinning NMR spectra of MoO3 catalysts supported on anatase and rutile polymorphs reveal the presence of two types of OH groups on the surface: OH groups of acidic nature and the basic OH groups. Temperature programmed reduction profiles of MoO2/TiO3 samples suggest that the reductability of MoO3 increases with Mo loading in the catalysts. the reduction of MoO3 to Mo proceeds in two stages and the hi The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicontinonitrile and are related to oxygen chemisorption sites on the surface.