A new electroacoustic instrument (AcoustoSizer, Matec Applied Sciences) has been used to measure the dynamic mobility of Nakaolinite and gamma-aluminium hydroxide (gibbsite) suspensions at MHz frequencies. In both suspensions the particles consist of thin, approximately hexagonal plates. Information about the particle size and the electrokinetic charge or zeta potential can be obtained from the dynamic mobility spectrum, For kaolinite particles, the interpretation of the dynamic mobility spectrum is complicated by the presence of anomalously high surface conductance. This profoundly affects the shape of the dynamic mobility spectrum at low (<5 x 10(-3) M) electrolyte concentrations, An approximate formula which takes surface conductance and particle shape into account has recently been derived by Loewenberg (1), Our measured dynamic mobility spectra are in good agreement with this theory. The surface conductance of the particles can be determined directly by dielectric response measurements on a high-frequency conductance bridge. The surface conductances determined in this way compare qualitatively with those needed to fit the dynamic mobility data using the theory described above.