We measured the dynamic mobility of a polystyrene latex at 1 MHz as a function of volume fraction using the ESA-8000. The volume fraction dependence is compared with a semiempirical equation as well as with some theoretical predictions. It turns out that our polystyrene latex exhibits a volume fraction dependence much weaker than that predicted by any of the theories. This suggests that (polystyrene) latices may not be the ideal model system and that the centrifugation process may influence the surface structure of the particles. We also measured the dynamic mobility spectrum of a silica sol as a function of volume fraction using the Acousto-sizer. The experimental spectrum of the silica sol was found to agree reasonably well with the semiempirical and theoretical spectra, especially below phi = 0.144. At higher volume fractions we observed positive phase angles that were not predicted by the semiempirical method nor the cell model.