Thermodynamic interactions in binary blends of a copolymer containing diene and olefin repeat units and a polyolefin were explored, through measurements of the effective binary Flory-Huggins interaction parameter, chi, and its temperature dependence, using small-angle neutron scattering (SANS). The first set of blends contained a copolymer of 1,2-polybutadiene (1,2-PBD) and poly(ethyl ethylene) (PEE, i.e., saturated 1,2-PBD or poly(1-butene)) blended with PEE homopolymer. The second set of blends contained a copolymer of 1,4-polyisoprene (1,4-PI) and poly(ethylene-alt-propylene) (PEP, i.e., saturated 1,4-PI) blended with PEP. The measured thermodynamic interactions for these blends were compared with the predictions from the random copolymer theory (RCT), employing the characterized chi parameter for the polydiene/polyolefin pairs (1,2-PBD/PEE and 1,4-PI/PEP). Experimental measurements were in excellent agreement with model predictions, implying that potential issues such as the presence of strong interactions and sequence effects on thermodynamic interactions are relatively unimportant in these model polydiene/polyolefin systems.