The equilibrium of gaseous H₂O-H₂ mixture with liquid iron-niobium alloys contained in alumina crucible shows that the activity coefficient of oxygen is moderately deminished by niobium. The oxide in equilibrium with the melt is not including iron and aluminum and considered to be NbO₂ (s). The equilibrium constant at 1600°C is [%Nb]·[f_O·%O]²=2·3·10^-4 where log f_O=-0.06.[%Nb]. The deoxidizing power of niobium is comparable to that of vanadium.