Wide-bandwidth measurements of ultrasonic velocity and attenuation have been made in a series of monodisperse suspensions of polystyrene in water and silica in water. Suspensions with three different particle sizes at fixed volume fractions were used in each case. The results enable comparison between theory and experiment of the variation in phase velocity and attenuation with frequency and with particle size. It is shown that there is good agreement between the attenuation results and the Allegra and Hawley theoretical model for both types of suspension investigated. However, in the case of the polystyrene suspensions, measured phase velocity as a function of frequency does not follow the expected R root f dependence, although the form of the measured dispersion curve is close to theoretical. Reasons for the discrepancies between the model and the experimental data are suggested, and it is concluded that the Allegra and Hawley formulation could form the basis of a particle sizing technique, provided that attenuation and velocity measurements were made over a broad, and continuous, frequency band.