This article introduces a newly built test rig with a vertical annulus test section with 20 mm outer and 10 mm inner tube diameter, for flow boiling experiments with highly viscous mixtures in downward flow. The technical features and the construction of this unique test rig are described. Results from an experiment are presented, where silicon oil with a viscosity Of eta(25 degrees C) = 1000 Pa s(-1) and pentane as solvent are used as a model system. The chosen parameters were a flow rate of 31 h(-1), a wall temperature of 120 degrees C and a pentane mass fraction of 9.4%. The experimental results were used for a first comparison with some models for flow boiling of highly viscous fluids.