The supersaturation ratio with respect to alumina precipitation, S_Al2O3^o(=(a_Al²·a_O³)_ss/(a_Al²·a_O³)_eq), in Fe-O-0.008∼0.090 mass%Al melt was electrochemically measured at 1873 K in an Al₂O₃ crucible by blowing CO₂ gas or adding an Fe-2 mass%Al alloy intermittently, using plug-type ZrO₂-9 mol%MgO and tube-type mullite probes. The S_ZrO2^o(=(a_Zr·a_O²)_ss/(a_Zr·a_O²)_eq) value in Fe-O-0.04 mass%Zr melt was also measured at 1873 K in a ZrO₂-11 mol%CaO crucible by adding an Fe-4 mass%Zr alloy. As a result, the critical values for log S_Al2O3^o and log S_ZrO2^o were 3.5 and 1.3, respectively. No supersaturation for the precipitation of SiO₂ was observed. The presence of solute element M (M=C, Mn, Cr, Si, and Ti) in an Fe-O-0.148∼0.158 mass%Al melt was found to significantly influence the supersaturation phenomenon.