The ultrasonic treatment of butyl rubber gum during extrusion with a grooved-barrel ultrasonic reactor was carried out at a mean residence time of 3.6 s and at different ultrasonic amplitudes. Gel permeation chromatography, NMR spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamic and mechanical property measurements were performed. The changes in the structure, curing behavior, and physical properties of the gum were found to be highly dependent on the applied ultrasonic amplitude. In particular, the molecular weight of the treated gum decreased and the molecular weight distribution increased with the ultrasonic amplitude. The number of double bonds in the ultrasonically treated gum was less than that in the virgin gum. The dynamic properties of the ultrasonically treated gums also indicated the occurrence of degradation during the ultrasonic treatment. The tensile strength and modulus of the vulcanizates prepared from the treated gums were reduced in comparison with those of the virgin vulcanizate because of degradation. In contrast, the elongation at break was higher. However, no significant changes in the thermal stability between the virgin and treated gums and among the vulcanizates were observed. (C) 2004 Wiley Periodicals, Inc.