High power ultrasound treatment of ethylene propylene diene monomer (EPDM) gum in extrusion process was found to affect its structure and properties. In comparison with the original EPDM gum, the ultrasonically treated samples were found to contain some amount of gel with sol having a higher molecular weight, possibly due to chain branching. Gel fraction and dynamic properties of the ultrasonically treated gums and their vulcanizates were measured as a function of processing conditions. Measurements of the stress-strain characteristics, dynamic properties, and hardness of EPDM vulcanizates prepared after treatment indicated a possibility of controlling their properties by varying processing parameters. (c) 2007 Wiley Periodicals, Inc.