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Defect structure of mechanically activated MoO 3 has been studied with the use of X ray diffrac tion, Raman spectroscopy, electron paramagnetic resonance, laser granulometry, and adsorption methods. Two stages of mechanical activation have been distinguished. At mechanical activation doses below 1 kJ/g, the fracture of oxide particles is the main process. At this stage, MoO 3 particle sizes decrease from 30 μm to 60 nm and specific surface area linearly increases to 30 m 2 /g, the sizes of coherent scattering regions decrease to 18 nm, paramagnetic centers are accumulated, and the Raman spectral bands corresponding to three dif ferent types of Mo-O bonds widen and shift. At doses above 1 kJ/g, the main process consists in the friction and aggregation of particles, which is accompanied by some reduction in the specific surface area and an increase in the particle sizes. At the stage of friction, the phase transition from an orthorhombic modification to a monoclinic modification of MoO 3 occurs seemingly due to a shift of one layer of the material in plane (100). The shift is accompanied by the accumulation of lattice microstrains in the same plane, formation of "stressed" Mo-O-Mo bridge bonds, and a substantial rise in the concentration of Mo 5+ radicals. The max imum total concentration of paramagnetic centers is 1 × 10 18 g -1 . It may be assumed that the radicals are formed due to the rupture of the most stressed molybdenum-oxygen bridge bonds.

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