Phosphorus partition ratios between CaO-Al2O3 and CaO-Al2O3-CaF2 fluxes and Fe-Csat-P alloys have been measured as a function of slag composition at 1500 °C. The effects of additions of BaO, Li2O, and Na2O to the CaO-Al2O3-CaF2 system on the phosphorus partition ratios at 1400 °C and 1300 °C have been measured. From the partition ratio, and assuming that the oxygen potential is controlled by C-CO equilibrium, the phosphate capacities of the fluxes were calculated. Also, the activities of Li2O and Na2O were measured as a function of slag composition at 1300 °C by equilibrating the flux and the metal with Pb-Li or Pb-Na alloy and CO in a graphite crucible. The results indicate that phosphorus partition ratios with carbon-saturated iron and the phosphate capacities for additions of more basic oxides decrease in the following order: Na2Oτ;Li2Oτ;BaO. The activities of Li2O and Na2O in calcium aluminate fluxes have large negative deviations from ideal behavior; the activity coefficients at infinite dilution are on the order of 0.05 and 10-5, respectively.