The goal of the current work is to develop a methodology to study the wetting behaviour of two immiscible liquids at high temperatures, and to investigate the parameters which influence the wetting properties. The wetting behaviour between synthetic FeMn alloy and synthetic slag has been investigated using the sessile drop technique. Two experimental procedures were implemented under both Ar and CO atmospheres: (a) FeMn alloy and slag placed next to each other on a graphite substrate; and (b) one droplet dropped on top of the other. FactSage is applied to calculate reactions and their equilibrium. The current work presents and demonstrates the suggested methodologies. The results indicate that the wetting between slag and FeMn alloy is relatively stable at temperatures up to 100 K above their melting points, regardless of the droplet size and atmosphere. MnO reduction is accelerated at higher temperature, especially in CO, thus increasing the wetting between FeMn alloy and slag, eventually fusing together. At even higher temperature, slag separates from FeMn alloy due to changing chemical composition during non-equilibrium MnO reduction.