AbstractDuring the last few years, improvements in control strategies became a necessity, with controls being targeted on material properties. The ever‐increasing demand for polymer blends spells the need for in‐line instrumentation capable of monitoring blend properties such as morphology. Since ultrasonics have proved to be sensitive to properties of multiphase materials, their performance for discriminating blend morphology was explored here. In‐line and off‐line (static, with no flow) ultrasonic measurements were made on different blends of polystyrene (PS), polyethylene (PE), and polypropylene (PP) for various viscosity ratios, and over the entire range of composition. Blends were compounded on a ZSK‐30 Werner&Pfleiderer intermeshing co‐rotating twin‐screw extruder. In‐line ultrasonic measurements were performed on both single‐screw and twin‐screw extruders. Ultrasonic properties (velocity and attenuation) were found to vary approximately linearly with composition. Mixing rules were evaluated for the description of ultrasonic behavior. For the attenuation measurements, the positive deviations from these rules were found significant. The additional attenuation was attributed to scattering generated by the presence of the min