Values of the bulk thermodynamic interaction parameter, chi(eff), for blends of anionically polymerized star (number of arms = 4, 6, 8, 12) and linear polybutadienes (PB) of well-defined architecture and molecular weight were measured as a function of temperature using small-angle neutron scattering. Comparison of these measured values of chi(eff) with results from comparable polystyrene (PS) blends suggests the existence of nonuniversal aspects in the thermodynamic interaction due to entropic contributions, chi(epsilon), arising from architectural differences in chains. While the value of chi(epsilon) for PS star/linear blends increases monotonically with number of arms in the star, the value of chi(epsilon) in the PB star/linear blends does not, a result which cannot be anticipated by the Gaussian field theory (GFT) of Fredrickson et al.(1) An important discrepancy between theory and experiment is also found for the variation of chi(epsilon) with linear chain length. Theory anticipates the value of x, should decrease with increasing linear chain size, but in fact it increases. Qualitative agreement with the GFT is found on two counts: chi(epsilon) decreases with increasing concentration of star (when assuming x(isotopic) for linear/linear blends is constant with concentration), and chi(epsilon) decreases with increasing length of the star arm. In general, the value of chi(epsilon) for a PB blend of star and linear components is larger than the value of x, for a comparable PS blend. Indeed, phase separation is observed in one particular PB blend of a six-arm star with a sufficiently large linear chain.