Gaseous sulfur monoxide (SO) is experimentally found to be a primary intermediate product in the oxidation of nonferrous metal and iron monosulfides. This finding is used to develop a theory for the chain oxidation of metal sulfides to form oxysulfide complexes MSO _x and gaseous SO. The main steps of the metal sulfide oxidation via a chain mechanism are thermodynamically characterized. The Gibbs energies of the oxidation reactions are shown to decrease in the absolute value relative to the type of sulfide as follows: FeS, Ni₃S₂, CoS, NiS, ZnS, PbS, CdS, CuS, and Cu₂S. The thermodynamic probability of all steps of the chain oxidation of the monosulfides with the evolution of gaseous atomic oxygen and sulfur is estimated.