This paper summarizes an extensive investigation of the thermodynamic interactions that govern phase behavior in blends of polyolefins and examines their relationship to pure component PVT properties. Interaction strengths, obtained by small-angle neutron scattering (SANS) measurements, were classified as regular or irregular according to their consistency with a solubility parameter formalism. Characteristic pressure P* and temperature T* were obtained from PVT data on the pure components with various liquid-state models. For the regular blends, a close correspondence was found between the SANS-based and PVT-based solubility parameter assignments, the latter being closely related to P*, as expected. The pattern of deviations for the irregular blends, positive in some and negative in others, effectively ruled out equation-of-state contributions as a general explanation. However, the results suggest that mismatches in both P* and T* play some role, and we offer some tentative attempts at generalization.