A broadband ultrasonic spectrometer has been used to measure ultrasonic attenuation and phase velocity dispersion as functions of frequency in kaolin suspensions over a range of solid volume fractions from phi = 0.01 to phi = 0.08 and over a pH range from 3 to 9. The Harker and Temple theory was used to simulate ultrasound propagation in the suspension, using measured slope viscosity, particle size, and size distribution. Simulated results for ultrasonic attenuation and phase velocity agree well with measured values. Both sets of results agree well and show that for volume fractions above phi similar to 0.05 attenuation and velocity dispersion increase for increasing flee size, whereas for volume fractions below phi similar to 0.05 attenuation and velocity dispersion both decrease. It is proposed that the mechanism for this change in behavior around phi similar to 0.05 involves changes in flee density and flee size distribution with phi and pH.