A model for the description of the viscosity of polymer/solvent systems made up of homologues, developed earlier, is generalized to normal polymer solvent mixtures. It is based on three premises: (i) the dissipation of energy takes place at the molecular interfaces; (ii) the friction between solvent and solute varies with composition due to a change in the flow mechanism (drainage of coils); and (iii) the specific coil volume generally also depends on polymer concentration. The resulting simple expression contains four system-specific parameters: a geometric factor gamma, which accounts for the differences of the surface to volume ratios of the components; a viscometric interaction parameter a, which measures the friction between solute and solvent in the case of fully draining polymer coils; [eta], the specific hydrodynamic volume of the polymer at infinite dilution (intrinsic viscosity), and [eta](Theta) the specific hydrodynamic volume under Theta conditions. The suitability of this model is demonstrated by means of extensive experimental data reported in the literature for the systems diethyl phthalate/poly(vinyl acetate) and diethyl phthalate/poly(methyl acrylate). It appears worthwhile to mention that the evaluation yields [eta] and [eta](Theta), even in the absence of information within the relevant composition range, and that there exists a linear correlation between gamma and alpha. (C) 2000 The Society of Rheology. [S0148-6055(00)00503-9].