Mutual solubilities and liquid-state interactions of polyisobutylene (PIB) with other polyolefins were investigated by a variety of techniques. Near room temperature PIB was found to be miscible with a limited range of polyolefin structures. The interactions for several blends in this range were determined by small-angle neutron scattering. The Flory-Huggins parameter chi was found to be large and negative at room temperature in all cases, contrary to expectations and to previous experience with blends of other saturated hydrocarbon polymers. However, chi diminished rapidly in magnitude with increasing temperature, eventually becoming positive and culminating finally in phase separation. Molecular weight independence of chi, volume change on mixing, glass transition temperature vs blend composition, and a phase diagram were established for one PIB/polyolefin system. The insensitivity of phase separation temperature to both blend composition and component molecular weight was shown to be a natural qualitative consequence of the unusually strong temperature dependence of chi. The volume changes and T-g behavior are also unusual and remain to be explained, along with the fundamental source of attractive interactions (negative chi) in blends of saturated hydrocarbon polymers.