The new iron(II)-thiolate complexes [((BIP)-B-iPr)Fe-II(SPh)(Cl)] (1) and [((BIP)-B-iPr)Fe-II(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO2H at a (His)(3)Fe-II center. Reaction of 1 and 2 with O-2 leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O-2, the spectroscopic and reactivity data, including O-18 isotope studies, are consistent with an assignment of an iron (IV)-oxo complex, [((BIP)-B-iPr)Fe-IV(O)(Cl)](+) (3), as the product of oxygenation. In contrast, 2 + O-2 results in direct S-oxygenation to give a sulfonato product, PhSO3-. The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O-2 activation. The thiolate ligands in 1 and 2 are essential for O-2 reactivity and exhibit an important influence over the Fe-III/Fe-II redox potential.