Isotopic polymer mixtures lack the structural asymmetries and specific interactions encountered in blends of chemically distinct species. In this respect, they form ideal model systems for exploring the limitations of the widely-used Flory-Huggins (FH) lattice model and for testing and improving new theories of polymer thermodynamics. The FH interaction parameter between deuterium-labeled and unlabeled segments of the same species (chi(HD)) should in principle be independent of concentration (phi), though previous small-angle neutron scattering (SANS) experiments have shown that it exhibits a minimum at phi approximately 0.5 for poly(vinylethylene) (PVE) and poly(ethylethylene) (PEE). We report new data on polyethylene (PE) as a function of molecular weight, temperature (T), and phi, which show qualitatively similar behavior. However, measurements on chi(HD)(phi) for polystyrene (PS) show a maximum at phi approximately 0.5, in contrast to PVE, PEE, and PE. Reproducing the concentration dependence of phi in different model isotopic systems should serve as a sensitive test of the way in which theories of polymer thermodynamics can account for the details of the local packing and also the effects of noncombinatorial entropy, which appear to be the main cause of the variation of chi(HD)(phi) for PE. These data also serve to quantify the effects of isotopic substitution in SANS experiments on polyolefin blends and thus lay the groundwork for definitive studies of the compatibility of branched and linear polyethylenes.