In an effort to improve our understanding of the structural changes during ultrasonic treatment, proton NMR transverse relaxation was used to study butyl rubber gum before and after ultrasonic treatment, and ultrasonically devulcanized unfilled butyl rubber. The relaxation decay was successfully described using a two-component model. The short component is attributed to entangled and crosslinked networks including the high molecular weight fraction of the sol, and the long component arises mainly from low molecular weight fractions of the sol. All component molecular mobilities and the relative contributions of the components to the echo were investigated. The present results were compared with our previous NMR studies of other devulcanized rubbers. (C) 2004 Elsevier Ltd. All rights reserved.