To recycle the iron, manganese, and lime in steelmaking slags, the removal of phosphorus is essential. Phosphorus is concentrated mainly in the dicalcium silicate phase in the slags. Previously, the slow cooling of a steelmaking slag in an air atmosphere was shown to cause the conversion of wustite to a strongly magnetic mg- mn-ferrite, and its close association with dicalcium silicate interfered with magnetic separation. The study aims at establishing the phosphorus mineralogy and its distribution in steelmaking slags by slow cooling under a neutral or somewhat reducing atmosphere and the methods of phosphorus removal. Magnetic separation produced iron-manganese-rich fractions analyzing 1.2 to 1.5 pct p2o5. Eighty percent of the phosphorus was associated with the locked dicalcium silicate, and a possibility of flotation upgrading is discussed.