The knowledge of residence time distribution (RTD) in industrial extruders is critical, notably when dealing with easily degradable polymers or when using extruders as chemical reactors. Many methods have been proposed for RTD determination but there are some drawbacks associated with each; they are expensive, hazardous, time consuming, or lacking sensitivity. A novel ultrasonic technique, sensitive to the filler concentration of polymer suspensions, is proposed. Ultrasonic properties (ultrasonic velocity and attenuation) were evaluated with regard to parameters such as linearity of the response, resolution of the measurement, and especially robustness to pressure and temperature variations. The attenuation, chosen for RTD evaluation along with a specific grade of calcium carbonate filler as the tracer, was then monitored to yield the RTD of the material in a twin-screw extruder, for different experimental conditions where quantity of filler utilized, as well as the method used to feed it, were changed in order to optimize the technique. (C) 1996 John Wiley & Sons, Inc.