Studies were made of the apparent melt viscosity and chain rupture of polyisobutylene melts during high mechanical shear at various temperatures, and for varied times of shearing action. A biconical rheometer was constructed and used for this study. It was observed that the torque developed in such a rheometer by the action of shear on molten polymer shows peculiarities. The initial high torque, or the apparent melt viscosity which may be derived from it, drops rapidly to a pseudo steady state value in about one or two minutes. An interval of shear cessation permits considerable return of the system to the original high value of the apparent melt viscosity, especially if the cessation interval is long (e.g., 10 min. at 45°C.). During the pseudosteadystate period there is a small and continuous drop in the already reduced torque or the apparent melt viscosity. Study of the chain rupture rate, made by examination of dilute solution viscosities, and the viscosity-average molecular weights determined from them, showed the rate of chain degradation to decrease as the temperature during the shearing operation is raised. The chain rupture rate decreases steadily as shearing is continued, and increases with increase of shear rate at a given temperature and time of shear application. The behavior of the apparent melt viscosity, decreasing sharply to a pseudosteady-state value shortly after intense shear begins, is presently pictured as due to two mechanisms, one chemical, the other mechanical, acting simultaneously. One process is considered to be the rapid breakdown of the chainlike molecules to smaller fragments forming mainly free radicals or ion pairs at their ends. These reactive ends begin immediately to recombine at a rate which increases with the rise in their concentration until the pseudoequilibrium state is reached. The second process is considered to arise from both chain disentanglement and the preferential migration of holes in the liquid away from the center of gravity of the chains to form regions of low viscosity which may be termed fault lines or regions.