Small-angle neutron scattering (SANS) measurements of interactions in polymer blends, chi(NS), generally depend on blend concentration phi, even though chi(NS) is evaluated with a model that assumes that the thermodynamic interaction parameter chi(FH)=chi(NS) is independent of phi. Londono et al. have reported chi(NS) to increase by similar to 4x when phi drops below 0.05 in polyethylene isotope blends. The relation between scattering and thermodynamics is addressed with incompressible Flory-Huggins theory wherein the thermodynamic interaction parameter chi may vary with concentration phi and degree of polymerization N; here chi(NS)(phi) not equal chi(phi), For polyethylene isotope and similar polyolefin blends, the strong upward curvature of chi(NS) implies a modest (ca. 30%) increase of chi. Macroscopic phase behavior is unaffected because the shape of the binodal remains essentially unchanged. The phi-dependence of chi(NS) in turn depends on N, leading to the following empirical expression for the thermodynamic interaction parameter : chi(phi, N) = beta - (2 gamma’/N phi(1) phi(2))(phi(1) ln phi(1) + phi(2) ln phi(2)). For polyethylene isotope blends at 155 degrees C, beta = 2.85 x 10(-4) and gamma’ = 0.15. Simple Flory-Huggins behavior with chi(FH) = beta is recovered when N approaches infinity. The source of the phi- and N-dependent second term is not known.