Based on technology described in a Du Pont patent for an off-line system, we have developed a prototype in-line ultrasonic cell for measuring particle size distribution. We have used this cell to verify the Allegra-Hawley model of ultrasonic attenuation in dilute(<5% vol) slurries of sub-micron ceramic particles, and we are developing a model that can cope with the multiple scattering effects occuring at higher concentrations. In this paper we present the results of attenuation measurements for ultrasound (2-50 MHz) in slurries with concentrations ranging from 0.5% to 38% (vol). The attenuation is proportional to slurry concentration up to 5% (vol) and is predicted by single scattering models. Above approximately 10% concentration, the attenuation is actually lower than expected. For the dilute slurries we find excellent agreement between our measurements and the Allegra-Hawley calculations, and the effects of the particle size distribution are evident in the ultrasonic attenuation spectrum. These ultrasonic data can be inverted to determine the particle size and concentration in aqueous slurries of sub-micron particles.