Consistent product quality requires the control of unsaturation in butyl rubber Analysis of the manufacturing process and comparison of laboratory and plant data led to the discovery ?hat the unsaturation of She rubber produced in the reactors can be predicted on-line based on chemical and reaction engineering principles The prediction equation uses the feed composition, the isobutylene concentration measured in the flashgas exiting the process, and the reactivity ratio of isobutylene and isoprene. The validity of the model was verified by comparing predicted unsaturation da ta to those calculated from mass balances, and to unsaturation data measured on rubber samples by 500 MHz NMR, epoxy titration, and the iodine index test.