Homogeneous mixtures of five poly(dimethylsiloxane) fractions [23-74 kg/mol] with CO2 were studied (100-700 bar and 30-70-degrees-C) in the entire range of composition. An antoclave, which allows the determination of densities, was used to prepare the solutions. Viscosities eta were measured in a rolling-ball and in a Searle-type pressure apparatus, respectively, depending on the polymer content. For the lowest molar mass sample (the only one below the entanglement value) the viscosity of the mixtures is always less than that calculated on the basis of volume fractions from additivity of the eta values of the pure components. With all other fractions this is only so for large polymer contents; as these solutions are diluted, the deviations become positive where the effects increase with rising M. This complex behavior can be described quantitatively by means of only one (physically meaningful and molecular weight independent) adjustable parameter, if one uses surface fractions as the composition variable instead of volume fractions. The evaluation of eta(T) at constant p with respect to activation energies E(double dagger) and of eta(p) at constant T with respect to activation volumes V(double dagger) reveals a pronounced sigmoidal dependence of these parameters on the composition of the mixtures. For the pure polymer one extrapolates the following values in the limit of infinitely long chains : E(infinity)double dagger = 75 kJ/mol at 300 bar and V(infinity)double dagger = 43 cm3/mol at 30-degrees-C. The measured densities yield maximum excess specific volumes on the order of -10 Cm3/kg; these effects are almost 10 times larger than predicted by the Flory-Orwoll-Vrij theory.