The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS-)-, sulfenate (RSO-)-, and sulfinate (RSO2-)-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO- species changes upon protonation as the S-O bond is elongated (by similar to 0.1 angstrom). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe-III in the active site of NHase as CysS(-), CysSOH, and CYSSO2- both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The preedge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe-III counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi* antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.