The phosphorus partition between the unkilled liquid crude steel and the high basicity (CaO/SiO2 = 4.2), basic oxygen furnace (BOF) slags with varying compositions of Al2O3, TiO2, and MnO is studied at temperatures 1600 and 1650 °C. The tests are conducted in both “slag-to-metal” and “metal-to-slag” directions and for durations of 30 and 60 min. The measured results are compared with the values reported in literature and found to be in good agreement with some of them. It is found in the investigation of high-basicity BOF slags, that Al2O3, MnO, and TiO2 lower the phosphorus partition. The phosphorus partition increases with increasing optical basicity. The phosphorus partition shows a maximum when the FeO content is in range of 25–30 mass%. The MgO content slightly increases the phosphorus partition. © 2021 The Authors. Steel Research International published by Wiley-VCH GmbH