Runaway reactions continue to occur in polymerization reactors, which are widely used in the chemical industry. Those of greatest concern include highly viscous multipbase mixtures. Considerable uncertainties exist in the design of emergency pressure-relief systems for such reactors because there are only limited experimental data available on the flow of such fluids in relief systems. Venting experiments are reported here that have been cat-tied out in a 350 L pilot-scale reactor for the runaway decomposition of an organic peroxide in xylene solvent, with batch volumes of 150 and 200 L and with a range of concentrations of polystyrene dissolved in the xylene to modify the viscosity. Measurements were made of pressures in the reactor, vent line, and catch tank; temperatures at different levels in the reactor and in the vent line and catch tank; viscosity in the reactor; mass in the catch tank; and two-phase density in the vent line. Analysis of the results shows that the venting behavior of a high-viscosity reaction system is complex. There is no simple relationship between maximum pressure and viscosity. The results indicate that the flow regime in the reactor is influenced by the fluid viscosity and that this is of primary importance in explaining the effects on maximum pressure. (c) Crown Copyright 2006. Published with the permission of the Controller of HMSO and the Queen's printer for Scotland.