Dynamically vulcanized PP/EPDM blends were treated by high-intensity ultrasonic waves during extrusion. These blends were compared with unvulcanized PP/EPDM blends that were treated by ultrasound during extrusion and then dynamically vulcanized. Dic pressure and power consumption were measured. The effects of different gap sizes, ratio of components, and number of ultrasonic horns were investigated. The rheological properties, morphology and mechanical properties of the blends with and without ultrasonic treatment were compared. The results obtained indicated that ultrasonic treatment induced then-no-mechanical degradation, causing enhanced molecular transport and chemical reactions at the interfaces, thus leading to in-situ compatibilization, which is evident by the morphological and mechanical property studies. Processing conditions were established for enhanced in-situ compatibilization of the PP/EPDM blends that were either originally dynamically vulcanized and then ultrasonically treated or first treated and then dynamically vulcanized. (C) 2004 Society of Plastics Engineers.