Autocausticizing, used for the recovery of kraft pulping chemicals, is nearing commercialization. This technology produces sodium hydroxide directly on dissolving the molten salts produced in the kraft recovery furnace in water. Thus, the dependence on the lime cycle for sodium hydroxide is reduced. Ongoing trials in several kraft mills are in progress to implement this technology. In this study, the reactions between sodium metaborate (NaBO2) and sodium carbonate (Na2CO3)-the anticipated reactants-were studied in a tubular reactor heated in an electric furnace. It was found that the reaction begins in the solid phase, followed by a liquid phase reaction. The reaction rate is extremely rapid once the system melts. It was proposed that these reactions could proceed through the intermediate formation of sodium diborate (Na4B2O5).