Iron silicate and iron sulfide liquids coexist in a large volume of the investigated system similar to the FeS-FeO-SiO 2 system (MacLean, 1969). Sodium is preferentially concentrated in the iron silicate liquid. Due to the strong attraction of Na 2 O to SiO 2 , the chemical potential of SiO 2 in melts is decreased considerably so that the phase relations are shifted, compared with the FeS-FeO-SiO 2 diagram, toward the SiO 2 corner. The troilite and wustite fields of primary crystallization expand into the two-liquid region. As a result, sulfide and/or oxide minerals appear earlier in the crystallization sequence than in the absence of soda, and the sulfide liquid disappears before the silicate liquid. This crystallization sequence is quite different from that of the Na 2 O-free system.Two invariant points are determined: two liquids + fayalite + wustite + troilite + iron + vapor, 877 degrees + or - 2 degrees C; two liquids + tridymite + fayalite + troilite + iron + vapor, 972 degrees + or - 2 degrees C. The latter point is outside the FeS-FeO-SiO 2 -Na 2 O system and is closely approximated within the system when this assemblage becomes univariant in the absence of metallic iron.As shown by MacLean (1969), oxidation of magma increases the size of the field of liquid immiscibility and hence fosters the separation of the sulfide liquid in nature. Na 2 O addition retards silicate crystallization facilitating gravitative separation and possible economic concentration of sulfide-oxide melts.