In most separation tasks in distillation, the liquid phase has a dynamic viscosity ranging from 1 to 10 mPa s under process conditions. However, in principle, it is also possible to distil mixtures with a viscosity that is two to three orders of magnitude above these values. One significant example is the removal of monomers from viscous polymer solutions by distillation. In contrast to evaporation technology, there is only little theoretical knowledge about the influence of viscosity on the separation performance in distillation tasks. Mass transfer correlations for calculating efficiency in random and structured packing columns were fitted predominantly with low-viscosity media and are therefore only of limited applicability for high-viscosity systems. In general. no experimental data are available for high-viscosity liquids that enable the development of such mass transfer correlations and the safe design of corresponding processes. The aim of the research work presented here was to answer the following questions: Which qualitative dependence exists between the viscosity and the separation performance of a distillation column? At which liquid loads can a distillation column be operated with high-viscosity media? On the basis of these results, distillation processes can be designed more exactly for high-viscosity applications and inadequate safety factors can be avoided. The investigations were carried out with a structured packing in a lab test column. The viscosity in the liquid phase achieved values up to 260 mPa s under process conditions.