The Fe-S-O melts were equilibrated with the CO-CO2-SO2 gases at 1473 K unde
r a S-2 (g) partial pressure of 0.0004 atm (=0.041 kPa). This S-2 isobar ru
ns on both sides of the pseudobinary FeSFeO system, thus adding a continuou
s extension to the isobaric network Of S-2 and O-2 in the literature. The a
ctivities of S-2 (g), O-2 (g), Fe (gamma), FeS (L), FeO (L), FeO1.33 (s), a
nd FeO1.5 (s) were calculated in the vicinity of the pseudobinary FeS-FeO m
elt. The activity of magnetite FeO1.33 (s) was observed to be greater than
that of FeO (L) in the ferrous binary FeS-FeO melts containing 35 to 55 mol
pct FeO.