The decrease of viscosity on shearing in a capillary at 37.8°C and a nominal rate of shear of 65 000 sec−1has been measured for 5, 10, and 15 weight‐percent solutions ino‐dichlorobenzene of 10 polyisobutylenes having viscosity‐average molecular weights from 40 000 to 2 300 000. Very little, if any, viscosity decrease was observed for polymers having molecular weights below 500 000, but marked decreases were observed for polymers with molecular weights above 500 000. The observed decreases were not recovered on standing and correspond to appreciable decreases in the molecular weights of the polymers. The molecular weight reduction can be explained on the basis of polymer chain rupture if the mechanical energy developed in a volume less than that enveloping a single polymer molecule can be concentrated into a single bond.