The oxygen potentials of several three- and two-phase equilibria in the Fe−Co−S−O quaternary system were measured atPSO2=1, 0.1, and 0.01 atm over wide temperature ranges. The measurements were carried out using a solid oxide electrolyte emf technique. The equilibria measured are sp+e+δ, s+e+ξ, sp+ξ+η, and sp+δ. The symbols sp, e, δ, ξ, and η denote the spinel, monoxide, monosulfide, metal sulfate, and Fe2O3 phases, respectively. Compositions for several of the equilibrated phases were measured using electron probe microanalysis. The present results and literature data for the constituent ternary systems were used to obtain thermochemical solution parameters for the sp, e, δ and ξ solid solution phases. The calculated potential-composition stability diagrams for SO2 pressures of 1, 0.1, and 0.01 atm at 973, 1023, and 1073 K, respectively, are in good agreement with the experimental results.