The additional coordination of inorganic sulfur or selenium to ditiothreitol (DTT)-iron complex induced a dramatic change of iron spin state from g=4.4 (high) to g=2.0 (low). The average g-values of DTT-Fe-S (1.99) and DTT-Fe-Se (2.02) complexes were larger than those of native S-adrenodoxin (1.97) and Se-adrenodoxin (2.00). The ability of O₂-uptake and activity of aniline hydroxylation increased in the order, DTT-Fe-Se>DTT-Fe-S>DTT-Fe complexes. In addition, the optimum pH regions for O₃-uptake and aniline hydroxylation by the model complexes were approximately parallel with those for complex formation in these DTT-iron systems. On the other hand, the hydroxylation activity of reduced S-adrenodoxin (yield of p-aminophenol : 10.90% per 90 min.) was clearly larger than that of reduced Se-adrenodoxin (7.64%). The effect of DTT-Fe-S and DTT-Fe-Se complexes on the activity of aniline hydroxylation, was compared with that of adrenodoxin and its selenium derivative.