The value of the thermodynamic interaction parameter, chi(eff), for star-star polybutadiene blends was determined with small-angle neutron scattering. Blends in which the stars have the same number of arms and blends in which the stars have different numbers of arms are investigated. For star-star isotopic blends with components having the same number of arms, the presence of the junction point of the star leads to a value of chi(eff) that is larger than that for an analogous linear-linear isotopic blend. However, changes in the value of chi(eff) resulting from small dissimilarities in the number of arms of the two components in the isotopic star-star blends were too small to resolve.