The thermodynamic behavior of linear and of branched oligo(dimethylsiloxane) (O-DMS) solutions was studied by means of vapor pressure measurements and vapor pressure osmometry at different temperatures for the thermodynamically favorable solvent THF. The branched material required for that purpose was synthesized and afterwards fractionated by means of the single solvent acetone to eliminate components of low degrees of branching. The Flory-Huggins interaction parameters, chi, for the systems THF/O-DMS as a function of composition pass a minimum at all temperatures (25, 40, and 60 degrees C) in the case of the branched material. For the unbranched oligomer such a minimum is only observed at 60 degrees C. At 40 degrees C the results are ambigous whereas the dependence is definitely linear at 25 degrees C. This exceptional behavior of the linear product at the latter temperature is tentatively attributed to the formation of favorable orientational order in the pure state under these conditions. At high oligomer concentrations THF interacts more favorably with the branched material, however, this preference is reversed upon dilution. All measured composition dependencies of chi can be modeled quantitatively by an approach accounting for chain connectivity and for the ability of the oligomers to change their conformation upon dilution. (C) 2010 Wiley Periodicals, Inc. J Polym Sc) Part B: Polym Phys 48: 1309-1318, 2010