The productivity of steelmaking processes, including production and refining of liquid steel, depends on the mass transfer rates. Due to the high temperatures involved in the processing of liquid steel, the rate controlling step of the processes is usually a mass transfer step. In steelmaking operations, different situations of mass transfer can occur, depending on the phases involved, liquid-liquid mass transfer, in the case of reactions involving liquid steel and slag; liquid-gas mass transfer, when a gas is injected into or onto liquid steel; liquid-solid mass transfer, when solid particles are injected into liquid steel to promote refining reactions. In all these situations, the evaluation of the mass transfer coefficient and the identification of the factors that affect its value are very important tasks. Different techniques have been developed to evaluate the mass transfer coefficient. These techniques vary according to the phases involved in the system being analysed. In the present chapter, the mass transfer coefficients will be defined. Then, the different techniques that have been adopted to evaluate the mass transfer coefficient will be presented and briefly discussed. Finally, a case study, analysing the mass transfer rate during decarburization in the RH degasser, will be described and discussed.