Slags are molten oxides formed during the high temperature smelting processes used to produce metals. These complex liquid oxide phases play a key role in removing unwanted elements from the several hundred million tonnes of metal produced annually. Understanding and controlling the flow behaviour of the slag phase is crucial to improving the technical and economical efficiencies of these operations. One of the key properties of slags is viscosity.Both fully molten and partially crystallised slags are encountered in the metallurgical industry. The rheological properties of these complex oxides can influence the flow patterns and heat transfer in the furnace, the ability to tap the slag from the reactor, the efficiency of separation of slag from metal and matte phases, the extent of slag foaming, the performance of continuouscasting fluxes, and the kinetics of metal smelting and refining reactions. Most operations operate close to the slag liquidus. Through improved knowledge of the rheology of fully and partially molten slags it may be possible to operate at lower process temperatures, which would greatly reduce the cost and the energy consumption of the process.The purpose of this thesis is to identify the properties that are important to the rheology of slags by assessing the following hypothesis:The rheological behaviour of high temperature heterogeneous slags is dependent upon particle size, particle size distribution, particle shape and volume concentration.A Brookfield DV-III+ rotating bob viscometer was used to measure the viscosity and flow behaviour of solid/liquid suspensions at 25oC analogous to those seen in the metallurgical industry. Suspensions of glass spheres and rods in glycerol were studied, having particle sizes up to 250 microns, and solid volume concentrations up to 40%. The effects of solid volume concentration, particle size, particle size distribution and particle shape have been measured.The viscosity of solid/liquid suspensions is dependant upon the solid volume concentration. For each particle size measured, it was found that the viscosity increased with increasing solid volume concentration. The scale of this increase was found to be greater as the solid volume concentration increased.