Carbonate treatment was tested as a means to mitigate the generation of alkaline leachate from basic oxygen furnace (BOF) slag. BOF slag was treated with 0.1, 0.5, and 1.0 M concentrations of NaHCO solution for 48 h at a liquid/solid ratio of 5 L/kg. At 1.0 M NaHCO, the pH of the leachate decreased from 12.0 to 11.3 because less free CaO was dissolved from the treated slag. Approximately 1.59 mg-Ca/g-slag of free CaO was dissolved from the untreated BOF slag while only 0.06 mg-Ca/g-slag was liberated from the treated slag. When the data from X-ray photoelectron spectroscopy and thermogravimetric analysis were taken together, formation of CaCO precipitates on the surface of the treated BOF slag was evident. Surface precipitation of CaCO was more pronounced when CO gas was used as an alternative carbonate source. Carbon dioxide treatment further decreased the leachate pH to 8.3, probably because it liberated more Ca from BOF slag during the treatment than 1.0 M NaHCO solution due to the pH difference (pH 6.6 and 9.6, respectively), in turn generating more CaCO precipitates. Scanning electron microscopy analysis revealed that more CaCO was precipitated on the CO gas-treated slag surface than on the NaHCO-treated slag. This study identifies the leachate pH reduction-mechanism and the effect of carbonate source which are expected to contribute to the environmentally safe management of BOF slags.