We have used H-1 NMR relaxation and pulsed-gradient spin-echo diffusion, C-13 NMR relaxation, and quantitative bulk measurements to characterize molecular mobility in unfilled butadiene rubber before and after an intense ultrasound treatment. In an effort to improve our understanding of the treatment process, we have performed the characterization at the molecular level. The H-1 NMR T-2 relaxation decay has been successfully described with a three-component model, which shows the evolution of the proportions and intermolecular mobilities of the changing spectrum of the motional species. The intramolecular mobility of the carbon backbone and structure has been examined with C-13 NMR, which shows significant isomerization resulting from the ultrasound treatment. The diffusion rates of the ultrasonically treated samples reflect the measured changes in the molecular weights of the Sol fractions. This investigation is the first to characterize the details of the delicate balance of the crosslinking, backbone modification, chain scission, and network degradation as an ultrasound treatment progresses. (C) 2004 Wiley Periodicals, Inc.