It was investigated if empirical models from the literature are capable to predict the two-phase pressure drop with highly viscous liquids in vent lines behind polymerization reactors or for production applications. The experiments were performed with upward pipe flow of nitrogen and water-Luviskol (R) solutions with varying dynamic viscosities. By measuring the difference of the static pressure in a vertical pipe across a test section, the gradient of the friction pressure was determined. The frictional pressure gradients show some unexpected scatter and even negative gradients were found in case of slug flow. A comparison with eight models shows for three of them a sufficient accuracy for the measured two-phase flow data with highly viscous liquids if the empirical parameters in the models are adapted to the new measurements.