Thermodynamic interactions in mixtures of two homopolymers and a block copolymer were obtained from small-angle neutron scattering (SANS) measurements. Experimental SANS profiles from homogeneous, ternary mixtures of model polyolefins-poly(ethyl butylene)/poly(methyl butylene)/poly(methyl butylene)-b-poly(ethyl butylene)-were compared with theoretical predictions based on the multicomponent random phase approximation (RPA). The polymers were nearly monodisperse and were synthesized by saturating the double bonds in anionically synthesized polydienes with H-2 and D2, thus yielding polyolefins with neutron scattering contrast. The theoretical scattering profiles depend on 16 structural parameters and six Flory-Huggins interaction parameters chi, all of which were obtained independently. The chi parameters were obtained from SANS measurements on binary poly(ethyl butylene)/poly(methyl butylene) blends. The SANS profiles obtained from the ternary blends were in quantitative agreement with theoretical predictions over the observable q range (0.01-0.08 angstrom-1) and over a wide range of block copolymer concentrations (from 10 to 80 vol. % copolymer). This suggests that the measured chi parameters are consistent with the original ideas of Flory and Huggins that chi is a measure of monomer-monomer interactions, and is thus independent of block copolymer concentration. Partial structure factors, related to correlations in the concentration of individual components, were examined by selective labeling and were also found to be in quantitative agreement with the predictions of the multicomponent RPA.