The architectural effects of branch points and chain ends on the bulk thermodynamic interaction parameter, chi(eff), in binary blends of branched polymers with their linear analogues have been investigated using small-angle neutron scattering (SANS) and a series of well-defined, regularly branched polystyrenes with the same molecular weight and differing only in the number of branch points or chain ends. The value of chi(eff) increases as the number of branch points increases for constant number of chain ends or as the number of chain ends increases for constant number of branch points. A Gaussian field theory for an athermal blend predicts the general trend in bulk chi(eff) with the number of chain ends but does not capture the changes in chi(eff) with changes in the number of branch points. Differences in chemistry among the various branch points probably have to be included to quantitatively predict changes in chi(eff) for shorter branches. Qualitative variations in the size of the branched molecules in solution compare favorably with a Gaussian model for the branched chain's radius of gyration. The variation in the average statistical segment length with chain architecture was also determined from the SANS data.