The effects of ultrasound treatment on the mean particle size, crystal structure, crystallite dimensions and specific surface area of natural muscovite and biotite samples have been investigated. Sonication of macroscopic flakes of muscovite and biotite produced a drastic particle-size reduction. The conditions for the preparation of micron and submicron-sized muscovite and biotite particles of narrow particle-size distribution by sonochemistry are described. The effect of sonication on particle-size reduction is more significant for muscovite than for biotite. Thus, for long sonication times (100 h), submicron and micron particles are predominant in muscovite and biotite, respectively. The resulting materials are crystalline, as assayed by X-ray diffraction, only broadening of the diffraction lines due to size-reduction was observed. Nuclear magnetic resonance studies revealed that the coordination of Al and Si was not modified by the treatment. Chemical analysis showed that the composition of the sample was not affected by the sonication except for a small contamination by Ti from the tip cup of the sonication instrument.