Seven θ-solvents were found for a sample of commercial butyl rubber which contained 1.40 mol-% of isoprene units. Two or three sharp fractions of this rubber sample in a series of solvents, including some of these θ-solvents, were examined by light scattering and viscometry in order to obtain data for the interpenetration function Ψ and the viscosity-radius expansion factor αη over a wide range of the statistical-radius expansion factor αs Plots of Ψ against αs3 were in good agreement with the “best-fit” curve from the authors’ previous data and those of Yamakawa et al., for other polymer–solvent systems. However, because of scatter, no conclusive information was obtained about the behavior of Ψ in the region of large αs3. Double-logarithmic plots of αη3 and αs3, although fitted approximately by a single curve, showed a slight but systematic deviation from the curve which had been proposed previously by Norisuye et al., on the basis of published data for a number of polymer–solvent systems and later confirmed by Yamakawa, et al., and also by Matsumoto, et al., by measurements on other polymer systems. However, it was clear that the authors’ data did not obey the familiar Flory–Fox relation. Finally, the values of β (binary cluster integral for unbonded segment–segment interaction) were computed from the observed αs by making use of the Fujita–Norisuye approximate relation for αs as a function of the excluded-volume parameter z.