The interaction of linear and of three-arm star polybutadiene (PB) with THF was studied in the temperature range between 25 and 55 degrees C. Information for dilute solution rests on light scattering experiments; Flory-Huggins interaction parameter chi stem from vapor pressure measurements as a function of phi, the volume fraction of polymer. Despite the minute divergence in the architecture, the second osmotic virial coefficients of the two PBs differ noticeably. The present work demonstrates that these disparities become much more pronounced as phi increases and that they depend strongly on temperature. These findings are interpreted on the basis of an approach accounting for the effects of chain connectivity and for conformational changes of the components. In this manner it is possible to rationalize the observed complex dependencies chi (phi, T), and to relate these effects to the dissimilar free volumes of the polymers.