Abstract: The defined oxidation ability of metallurgical slag based on the ion and molecule coexistence theory (IMCT), i.e., the comprehensive mass action concentration of iron oxides N_FetO, was verified by comparing the calculated N_FetO and the reported activity of iron oxides αFetO in selected 14 slag systems. To calculate N_FetO in the selected slag systems, a thermodynamic model for calculating the mass action concentrations of structural units in CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ type slag systems, i.e., the IMCT-N_i thermodynamic model was developed. N_FetO in the other 13 slag systems can be obtained by simplifing this IMCT-N_i. It is shown that the defined comprehensive mass action concentration of iron oxides N_FetO is more accurate than the measured activity of iron oxides a_FetO in characterizing the oxidation ability of the selected Fe_tO-containing slag systems.