The phosphorus partition (LP) and phosphate capacity CPO34 were measured for slags in the CaO–SiO2–MgO–FetO–(MnO–Al2O3–TiO2–P2O5) system over temperatures representative of basic oxygen steelmaking. The measured LP values were found to be in good agreement with those predicted from the model of Assis et al. The oxygen potential of the slag-metal systems studied was also evaluated and used in combination with the measured LP to calculate the CPO34− of the slags. Capacity values in the range of 7.77×1016 to 4.27×1019 for temperatures 1 550°C to 1 700°C were obtained. Correlations of CPO34− with common measures of slag basicity (v-ratio and optical basicity) were sought and reported. Consistent with other researchers it was found that the LP and CPO34− increased with increasing slag basicity and decreasing temperature.