Factors influencing the slag equilibrium phosphorous distribution ratio, L-p, and the phosphide capacity, C-p, of ferromanganese smelting slags under highly reducing conditions were studied at 1500 C. The investigations were conducted by equilibrating slags with the respective ferromanganese alloys contained in graphite crucibles under CO and Ar-CO atmospheres at oxygen partial pressures of 1.22 x 10(-16)-7.83 x 10(-19) atm at 1500 degrees C. The number and composition of the slag samples were determined using the extreme vertices design method for mixture experiments in order to optimize the experimental work. Analytical results of the final equilibrium slag and metal samples were employed to calculate the L-p and C-p of SiO2-Al2O3-CaO-MgO-MnO-P2O5 slags in equilibrium with Mn-Fe-Si-C-sat-P metal phase. The results were further used to develop empirical models to predict the distribution ratios and phosphide capacities as a function of slag composition. It was established that the L-p and C-p parameters increased by increasing slag basicity, CaO content and increasing oxygen partial pressure. (c) 2007 Elsevier Ltd. All rights reserved.