Mechanically activated MoO3 has been characterized by infrared and Raman spectroscopy. A drastic decrease in the Raman intensity connected with particle size reduction is observed when MoO3 is ground in a planetary mill for 600 min. The broadening of Raman bands is linearly correlated with the increasing BET surface area. The observed changes in Raman intensity ratios are related to the known lattice contraction and expansion of MoO3-x and thus arise from known distortions in suboxides. Mechanical activation induces only minor internal. lattice strain; thus, only small shifts of the bands assigned to rigid chain modes are observed. New small bands observed below 50 cm(-1) are attributed to backfolded phonon modes due to the formation of shear superstructures, which were detected by XRD and HRTEM. A resonance effect is indicated by a shift of these bands upon changing the excitation wavelength from 457.9 to 487.9 and 514.5 nm, respectively. This resonance effect is confirmed using a laser line at 621.9 nm, which results in a much broader Rayleigh wing, and a multitude of bands below 116 cm(-1), a reversed intensity of the pair of wagging modes at 283/290 cm(-1), and additional shoulders at 621, 639, 990, and 1005 cm(-1). Further confirmation is found in a resonance Raman experiment, where the bands observed are suggested to arise from Mo5+=O stretching vibrations of defect sites, which were also detected by DR-UV/vis and ESR spectroscopies. The transmission IR spectra in the far-infrared region (200-35 cm(-1)) are affected in a complex way by particle size reduction and the increasing influence of grain surfaces. This behavior of the FIR bands is connected with the complex, stepwise particle size reduction revealed by XRD. The observed spectral changes in the far-infrared (450-200 cm(-1)) and mid-infrared (1200-450 cm(-1)) regions as compared to single-crystal data, are explained by TO-LO splitting of the B-3u, modes. DRIFTS is found to be more sensitive toward spectral changes due to LO-TO splitting and toward minority species, like molybdenum hydrates, as compared td transmission IR spectroscopy, probably due to the higher surface sensitivity of this technique. Combination modes above 1010 cm(-1) lose intensity upon particle size reduction, thus reflecting the destruction of the MoO3 lattice. A drastic increase in the intensity of bands that are assigned to OH vibrations indicates the presence of water interconnected by H bonds in microcrystalline MoO3. The detection of the OHO deformation mode at 1425 cm(-1) and additional signals at 770 and 955 cm(-1) reveals an increasing formation of molybdenum hydrates upon mechanical activation.