Rapid, on‐line determination of particle size and concentration is required for the efficient process measurement and control of many processes in government and industrial applications such as waste remediation for the Department of Energy sites and process control for chemical and pharmaceutical manufacturing. However, existing methods based on ultrasonic attenuation can become inaccurate for highly concentrated suspensions due to careful transducer alignment and the complicated mathematics required to describe multiple scattering, which controls the attenuation. Two measurements that help to overcome these difficulties are the ultrasonic backscattering and diffuse field. Backscattering is attractive because the single scattering theories typically used to describe backscattering are mathematically simpler than attenuation theories and lend themselves to more stable inversion processes. Also, the measurements of backscattering and diffuse fields do not require long travel distances and can be made with a single transducer thus eliminating alignment problems. We will present ultrasonic measurements on solid liquid suspensions designed to elucidate the particle size and concentration at high concentrations. © 2003 American Institute of Physics