In this paper the performance characteristics, expressed as cut-size, grade-efficiency curve and pressure drop, have been determined for a hydrocyclone acting on highly viscous liquids. These results are of high quality and can serve as a benchmark for simulations and model predictions. To understand the results, CFD, large eddy simulations of the flowpattern and pressure drop have been carried out, among other things to determine the vortex intensity as a function of liquid viscosity. A version of a main stream cyclone model has been identified that predicts the trend in hydrocyclone performance with liquid viscosity well, both in terms of separation efficiency and pressure drop. This model is given in full, providing a valuable tool in evaluating the potential of hydrocyclone technology for new and unconventional separation duties. (C) 2014 Elsevier Ltd. All rights reserved.