<p>Pyrometallurgical treatment based on carbothermal reduction enables an efficient recycling approach for phosphorus-containing basic oxygen furnace slag. This would allow the recovery of iron and chromium as well as critical raw materials by the classification of the European Commission, manganese, and the non-metal phosphorus, in a specifically designed reactor concept. In addition, the remaining oxidic components could be utilised as a secondary raw material in the cement industry. The difficulty in the carbothermal reduction of oxidic phosphorus compounds is that highly reactive gaseous phosphorus is inevitably formed. Therein the main challenge represents the avoidance of undesirable phosphide formation, as molten metal and gaseous phosphorus exhibit high chemical activity. In basic oxygen furnace slag, the quantity of transition metals can be determined in the following order: iron first, followed by manganese and chromium. However, during carbothermal reduction, all transition metals tend to form phosphide species. To investigate this behaviour, basic oxygen furnace slag modified with quartz sand and a synthetic replica were carbothermally reduced at 1873 K and the obtained metal phase was subsequently analysed for the presence of phosphides. Additionally, a synthetical produced phosphorus-rich slag from a hypothetical process route is comparatively investigated. The experimental results have shown that phosphorus during carbothermal reduction of basic oxygen furnace slag has a high chemical affinity to all transition metals present in the slag, resulting in the formation of mixed phosphide species. A deeper understanding about the mechanism of phosphide formation during carbothermal reduction of basic oxygen furnace slag facilitate the definition of parameters for an enhanced dephosphorisation rate and an improved metal quality.</p>