Small-angle neutron scattering (SANS) studies of binary mixtures provide chi(NS), a measure of thermodynamic interactions between dissimilar polymer chains, one of which is usually labeled with deuterium. For polymers differing only in isotopic substitution (isotope blends), chi(NS) is Seen to diverge strongly upward (or sometimes downward) at low concentrations of either blend component. This concentration dependence seems to vanish in the limit of large degree of polymerization N. Experimental results can be described by chi(NS)(phi,N) = beta + gamma/N phi(1 - phi), where phi is the volume fraction of deuterated polymer. For SANS from a series of blends with different phi it is shown that systematic errors in N and/or the static structure factor S(0) lead to precisely the same chi(NS)(phi,N) when the Flory-Huggins interaction parameter chi is constant. While results for some isotope blend systems can be accounted for with reasonable error estimates, others appear to have a real dependence of chi(NS) on phi and N. It is suggested that these "non-Flory-Huggins" effects stem from a modified entropy of mixing that is most evident in dilute blends. The concentration dependence of chi(NS)(phi) has no practical effect on macroscopic phase behavior.