A novel sonochemical approach for the preparation of molybdenum oxide (Mo2O5·xH2O) and molybdenum carbide (Mo2C) clusters coated on silica carriers, in which Mo(CO)6 precursor serves as an in situ source for the coating phase of both materials (molybdenum oxide and molybdenum carbide), has been described. Ultrasonic irradiation of a slurry of molybdenum hexacarbonyl, Mo(CO)6, and silica microspheres in decane for 3 h, under ambient air and argon, yields diphasic molybdenum oxide-silica (MOS) and molybdenum carbide-silica (MCS) composites, respectively. Characterization using powder X-ray diffraction and transmission electron microscopy, with selected area electron diffraction, shows the amorphous nature of the nanocomposites. The phase evolution by long-term thermal reduction of MOS under H2 shows that the Mo2O5 undergoes stepwise reduction following the sequence, MoV → MoIV → Moo. The TEM image of MOS and MCS shows that the sonochemical decomposition products of Mo(CO)6 attached on the silica carrier as clusters, as thin layers, or as nanoparticles depends upon the precursor composition. UV-visible absorption studies on the sonochemically produced MOS demonstrate that the characteristic absorption of the MoV (d1 cation) oxide system and the Mo ions are likely to possess two types of coordination symmetry (Td and Oh). Considerable changes in the characteristic UV-visible absorption of MOS, compared to that of bulk molybdenum oxide and bare silica, was observed, perhaps associated with the chemical interaction, to form a desirable interfacial bond. FT-IR spectroscopy illustrated the structural changes that occur when the amorphous SiO2 is coated sonochemically. It has been proposed that the coating takes place via ultrasonic-cavitation-induced decomposition of the precursor into the required coating phase and the breaking of the strained siloxane link of the silica; subsequently, the coating phase and silica particles collide with each other and deposit themselves into the coating.