Methanobactin is a 1217.2 Da polypeptide intimately involved in the function of methane monooxygenase (MMO). Methanobactin's role appears to be that of a “chalkophore” in which extracellular copper ions are bound, transported to the cell interior and either stored or transported to MMO. However, the methanobactin/Cu complex also appears to be directly involved in the function of MMO, although its role is still unknown. We have examined the binding of Cu(II) ions with methanobactin by X-ray photoelectron spectroscopy and find that Cu(II) reduces to Cu(I) upon binding to methanobactin, and that S atoms associated with methanobactin's thiocarbonyl groups are involved in the binding. These results have implications about how Cu ions bind to methanobactin and the stoichiometry present.