The mass transfer rate during ladle refining was quantified by taking sequential steel and slag samples during the treatment of 20 heats. Each heat was stirred with a different argon flow rate, ranging between 0 and 63 scfm. Heats were treated at two different plants. Al-killed steel was produced at an LMF in 151-t ladles. Si-deoxidized steel was produced at an LMF in 123-t ladles. Mass transfer rate constants were determined for each heat by using process simulation (Metsim) and thermodynamic (FactSage) models. Relationships between mass transfer rate constants and stirring powers as well as ladle geometries were compared between the two plants and published literature. It was found that the reaction kinetics during ladle refining depend on the bulk transport of the steel to the slag/steel interface and on the thermodynamic equilibrium at the slag/steel interface. The necessary refining time decreases if the newly-defined specific steel transport rate is maximized and the slag has a low basicity and FeO concentration before the start of de-S.