© ICS 2018 - 7th International Congress on Science and Technology of Steelmaking: The Challenge of Industry 4.0. All rights reserved. Phosphorus has a negative effect on the mechanical properties of steel. Lower quality ores are likely to lead to an increase in the phosphorus load of the basic oxygen steelmaking (BOS) process. This is likely to cause difficulties in achieving steel specification using current processing methods and conditions. Steel dephosphorisation requires slags with a high phosphorous partition (LP) and phosphate capacity (C PO4 3−). LP and C PO4 3− have been measured in the CaO-SiO 2 -MgO-FetO-(MnO-Al 2 O 3 -P2O 5 ) system at temperatures (1550°C to 1700°C) relevant to the BOS process. Correlations of C PO4 3− with common measures of slag basicity were sought and reported on. Capacity values in the range of 7.77x10 16 to 4.27x10 19 for temperatures 1550°C to 1700°C were obtained. Consistent with other researchers it was found that the LP and C PO4 3− increased with increasing slag basicity and decreasing temperature. There were little data available in the published literature at the higher temperatures experienced in a BOS vessel. The results reported on in this article represent an extension and augmentation of the current knowledge of the dephosphorisation behaviour of BOS slags.