To predict viscosities for complex polymer-solvent mixtures according to Eyrings kinetic theory of viscous flow, a suitable segmentation approach has to be applied. Reasonable segmentation approaches are elaborated and discussed in detail. Considering viscous flow as surface phenomenon, a new segmentation approach is developed based on molecular surfaces of each species in the mixture. For six binary polymer-solvent mixtures of different complexity, mixture viscosities were experimentally obtained and compared to model predictions. For all mixtures, the best agreement between model predictions and experimental data is obtained by using surface-based weighing fractions. Moreover, theoretical evidences for choosing surface-based weighing fractions are provided.