The transport properties of molten salts at high temperatures are crucial for the efficient design of industrial equipment and chemical processes involving these materials. However, large discrepancies among the data published by different authors and different laboratories still exist, mostly caused by the difficulties in accurate measurements. This situation with regard to thermal conductivity and viscosity is especially serious for plant design. The aim of this paper is to demonstrate the implications of the uncertainty of molten salt thermophysical properties, such as thermal conductivity and viscosity, on the design of heat exchangers and other equipment. Some of the recent applications of ionic liquids at high temperatures, both as heat-transfer and chemical-reaction media, will be presented in order to illustrate how the knowledge of thermophysical properties is important for proper and optimal technological design, using two examples (thermal storage in solar plants and molten salt oxidation of wastes) among the many possible. The results obtained support that the implementation of those applications needs a careful selection of experimental data; other-wise, equipment will be either under- or overdimensioned, with the consequent poor operation or increased capital costs.