Abstract The dependence of composition of the phases Ba 1+ x Fe 2S 4 on sulfur vapor pressure and starting composition was investigated at 650, 747, and 800°C. The infinitely adaptive series Ba 1+ x Fe 2S 4 spans the compositions 0.072 ≤ x ≤ 0.142. The value of x decreases as the sulfur vapor pressure increases at a given temperature. Varying the ratio of Ba Fe in the starting mixture has no effect on the Ba 1+ x Fe 2S 4-sulfur fugacity relationship. The phase BaFe 2S 4 is not part of the infinitely adaptive series.