When a chemical reaction occurs between two immiscible liquids, mass transfer is continuously taking place at the liquid-liquid interface. Several studies have shown that if the species being exchanged between the two liquids are surface-active, a very pronounced decrease in interfacial tension can occur which can lead to a phenomenon called spontaneous emulsification. In steelmaking this behavior has been observed for several reactions that involve the transfer of impurities from molten steel to a molten-oxide slag but little quantification has been made. This work focuses on spontaneous emulsification due to the dephosphorization of a Fe-P drop immersed in a Basic Oxygen Furnace (BOF) type slag. An Au-image furnace attached to a Confocal Scanning Laser Microscope (CSLM) was used to rapidly heat and cool the samples at different times and X-ray Computerized Tomography (XCT) was used to perform the surface area calculations of the samples where the slag/steel reaction was allowed to occur for distinct times. The results show that the surface area of the metal drop rapidly increases by over one order of magnitude during the first 60 seconds of the reaction while the chemical reaction is occurring at a fast rate. Once the reaction slows down, approximately after 120 seconds, the droplets coalesce back together minimizing the surface area and returning to a geometry close to its equilibrium shape.