Cox17 is the candidate copper metallochaperone for delivery of copper ions to the mitochondrion for assembly of cytochrome c oxidase. Cox17 purified as a recombinant molecule lacking any purification tag binds three Cu(I) ions per monomer in a polycopper cluster as shown by X-ray absorption spectroscopy. The CuCox17 complex exists in a dimer/tetramer equilibrium with a 20 μM k d. The spectroscopic data do not discern whether the dimeric complex forms a single hexanuclear Cu(I) cluster or two separate trinuclear Cu(I) clusters. The Cu(I) cluster(s) exhibit(s) predominantly trigonal Cu(I) coordination. The cluster(s) in Cox17 resemble(s) the polycopper clusters in Ace1 and the Cup1 metallothionein in being pH-stable and luminescent. The physical properties of the CuCox17 complex purified as an untagged molecule differ from those reported previously for a GST−Cox17 fusion protein. The CuCox17 cluster is distinct from the polycopper cluster in Cup1 in being labile to ligand exchange. CuCox17 localized within the intermitochondrial membrane space appears to be predominantly tetrameric, whereas the cytosolic CuCox17 is primarily a dimeric species. Cys→Ser substitutions at Cys23, Cys24, or Cys26 abolish the Cox17 function and prevent tetramerization, although Cu(I) binding is largely unaffected. Thus, the oligomeric state of Cox17 may be important to its physiological function.