Physicochemical changes induced in MoO3 by mechanical activation in a planetary mill were investigated. During the milling process, the BET surface area increases from about 1.3 to 32 m(2)/g. Energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) of the powdered MoO3 samples reveal that no agate is abrased during the milling process. An estimation of the mean particle size using the BET data or scanning electron microscopic (SEM) images indicates a decrease from about 1 mu m to about 50 nm. The primary crystallite size calculated from X-ray diffraction (XRD) line broadening also shows a decreasing size from about 160 nm to about 80 nm. The difference between the particle size of unmilled MoO3, as determined from the BET surface area or by scanning electron microscopy (SEM), and the calculated primary crystallite size using X-ray line broadening is explained by MoO3 particles consisting of smaller primary crystallites. The smaller average particle size of MoO3 milled for 600 min calculated from BET data, on the other hand, as compared to the XRD primary crystallite size is ascribed to the presence of ultrafine amorphous material which is X-ray amorphous and, therefore, does not contribute to the X-ray line broadening. This formation of amorphous material is also indicated by an increasing amorphous scattering background in the X-ray diffraction patterns. In SEM pictures, these particles appear to have an amorphous overlayer. The strange behavior of both the X-ray diffraction pattern quality and the diffuse scattering background, which do not coincide, during mechanical activation probably indicates a complex process of particle size reduction. Variation of X-ray reflection profiles, intensity ratios, and additional X-ray reflections may point toward the formation of shear defects during this process. A Warren-Averbach analysis of the most intense X-ray reflections of milled MoO3 reveals that internal strain is only marginally enhanced by mechanical activation.