The magnitude of the thermodynamic interaction in a bulk binary homopolymer blend due to regular branching alone has been estimated from small angle neutron scattering (SANS) measurements for the first time using blends of star and linear poly(styrene)s of well defined structure. The value of chi for a blend of deuterated four arm star (M = 100 000 g/mol) with hydrogenous linear polystyrene (M = 132 000 g/ mel) of comparable molecular weight is chi = 0.1278/T - 0.0002 over the temperature range of 120-225 degrees C for a segment volume of 100 cm(3)/ mel. The contribution to chi ascribable to architecture effects is just distinguishable from that due to isotopic substitution alone and in surprisingly good agreement with an approximate prediction from a mean field theory. The precise value of chi varies depending on which component is labeled.