Methods for measuring the thermal diffusivity, thermal conductivity, density and viscosity of molten slags and fluxes have been critically reviewed. The most common method for determining the viscosity of slags is the concentric cylinder method and in particular the rotating bob variant. There is now a well established reference material suitable for high temperatures and two standards for the most common materials investigated i.e. glass and mould powders. For low viscosities (of the order of 5mPa.s) the oscillating vessel method is suitable. The oscillating plate method has been investigated extensively for the measurement of mould fluxes by Japanese workers. A major problem is the containment of the slags to prevent unwanted reactions and the choice of materials used by a selection of workers is reviewed. Fewer density than viscosity measurements have been made. The most popular methods of determining densities are the maximum bubble pressure (also suitable for determining surface tension); Archimedean and sessile drop methods. There is no corresponding infra structure for density measurements of slags as for their viscosities. For the thennal diffusivity/conductivity, the most common methods are the laser flash and the transient hot wire techniques. The treatment of the experimental results is complicated, such as including the effects of radiation for translucent materials. It is clear, if the information is industrially important, there are substantial opportunities for further method development including standardisation and understanding the physical processes of heat transfer in these materials.