A combination of pulsed EPR, CW EPR, and X-ray absorption spectroscopies has been employed to probe the geometric and electronic structure of the E. coli periplasmic molybdenum-dependent methionine sulfoxide reductase (MsrP). 17O and 1H pulsed EPR spectra show that the as-isolated Mo(V) enzyme form does not possess an exchangeable H2O/OH- ligand bound to Mo as found in the sulfite oxidizing enzymes of the same family. The nature of the unusual CW EPR spectrum has been re-evaluated in light of new data on the MsrP-N45R variant and related small-molecule analogues of the active site. These data point to a novel "thiol-blocked" [(PDT)MoVO(SCys)(thiolate)]- structure, which is supported by new EXAFS data. We discuss these new results in the context of ligand-based and metal-based redox chemistry in the enzymatic oxygen atom transfer reaction.