The viscosity of polymer/solvent systems is modeled as a function of composition under the premises that the dissipation of energy is taking place at the molecular interfaces and that the friction between solvent and solute varies with composition due to a change in the flow mechanism (drainage of coils). The simple expression obtained in this manner contains three system-specific parameters: a geometric factor gamma, which accounts for the differences of the surface to volume ratios of the components; a hydrodynamic parameter alpha, which measures the friction between solute and solvent in the case of fully draining polymer coils; and beta, which corrects for changes in the friction between unlike molecules resulting from collective motions owing to limited draining. Experimental data published for 12 poly(dimethylsiloxane)/pentamer mixtures can be represented quantitatively by this relation; moreover the knowledge of the three system-specific parameters permits the calculation of intrinsic viscosities, and the molecular weight dependencies of gamma and alpha yield the entangle molecular weight of the polymer.