Thermodynamic interactions in binary blends of model polyolefins were investigated by small-angle neutron scattering (SANS), cloud-point determinations, and PVT measurements. Results for four microstructures-atactic polypropylene, head-to-head polypropylene, and two saturated samples of polyisoprenes with reduced 1,4 content-were incorporated into a solubility parameter (SP) scheme that had been developed earlier for various model copolymers of ethylene with propylene and l-butene. Component SP values were assigned, after confirming their uniqueness by various tests of internal consistency, with SANS-determined interaction strengths for their blends with other components. As found earlier, the SANS-based assignments agree rather well with independent estimates of SP from the pure component PVT properties. Several new instances of irregular mixing-blends for which the interaction strength departs significantly from the SP prediction-were identified, and some apparent patterns were noted. The suggested relationship between interaction strength and mismatch of component segment lengths was also examined in some detail. What we find is an interesting global correlation between segment length and SP assignment, whether obtained from SANS data or inferred from pure component PVT properties, but no connection at all between segment length mismatch in blends and either the temperature dependence of interaction strength or the presence of mixing irregularity.