An investigation has been made of the degradation of Vistanex polyisobutenes having viscosity‐average molecular weights from about 500 000 to 2 500 000 by shearing in solution in cetane at polymer concentrations from 5 to 20 weight percent, at nominal rates of shear from 9000 to 66 000 sec−1, and at temperatures from 20° to 80°C. The initial apparent shearing loads, observed at the beginning of the degradation process, vary in unexpected ways with variations in nominal rate of shear, temperature, concentration, and initial molecular weight of the polymer. In some cases the observed variations are opposite to those which would be expected on the basis of viscoelastic resistance to shearing. These results can be explained on the basis of the hypothesis that the observed apparent shearing forces include some contribution associated with the energy used up in breaking chemical bonds in the polymer molecules during the degradation process. If this hypothesis is correct, valid viscosity values cannot be obtained directly from observed apparent shearing forces in viscometry measurements under conditions where shear degradation occurs.