Ultrafine MoO3 and WO3 particles have been prepared in a flame reactor. Optimizations of reactor parameters allowed one to obtain high surface area (greater than or equal to 40 m(2) g(-1)) orthorhombic MoO3 and monoclinic pseudo-orthorhombic WO3, after a calcination in air at 673 K to eliminate residual chlorine, Platinum loaded (0.1-0.2 wt.-%) catalysts supported on these oxides were characterized by volumetric measurements of H-2 consumption, CO chemisorption, transmission electron microscopy and X-ray diffraction (XRD). In the presence of H-2, hydrogen bronzes of MoO3 and WO3 were formed by the so-called hydrogen spillover process. In the range 298-673 K, the value of x in HxMoO3 bronzes lay between 1.44 and 2.10 whereas that of y in HyWO3 ranged from 0.44 to 0.58. Platinum was well dispersed on MoO3 or WO3 and a partial reduction of the supports as a result of H-2 treatments at 673-773 K was shown by XRD.