A fluid–fluid mass transfer model implemented in computational fluid dynamics is presented. This includes the model description, model validation, and application of a liquid steel/slag mass transfer within a purged steel ladle. The model considers the prevailing flow and concentration. For the flow, it distinguishes between laminar and turbulent flows. In the case of turbulent flow, the boundary layer at the liquid–liquid interface is modeled analogous to the wall treatment for species. Interface concentrations were determined using the diffusion equation. Unlike other presently available mass transfer and reaction models, the established model aims to calculate the mass transfer coefficient depending on the local fluid flow and concentration conditions. For a case without convection, the model was tested using a comparison with the solution of the one-dimensional diffusion equation. In the ladle case, the mass flux density was compared with data from the literature. The model was applied to the steel/slag mass transfer in a gas-purged steel ladle. It was found that the mass transfer increased with increasing purging rates within a certain range, increasing slag bath heights, and decreasing viscosities.