The behavior of three different high molecular weight polyisobutylenes (PIB) in solutions of hexane and cyclohexane at 30°C has been investigated by viscometry. Mark–Houwink relations have been examined, the polymer–solvent interaction is discussed in terms of the calculated Huggins constant,k; parameteraof the Mark–Houwink equation; and equivalent hydrodynamic volumesVe. The molecular weights of the three sample of polyisobutylene are remarkably well fitted with the intrinsic viscosity data.