This chapter focuses on three proteins: (1) rubrerythrin (Rr), (2) desulfoferrodoxin (Dfx), and (3) an [Fe—S] protein containing a six-iron cluster. These three proteins display the presence of redox metal centers with metal compositions spanning from mono- through bi- to hexanuclearity. Rr was first characterized from Desulfovibrio vulgaris (Hildenborough) (DvH). All of its purification procedures are performed at 4〫 and pH 7.6. The presence of Rr is judged by a change in absorbance at 490 nm after ascorbate reduction. Rr is composed of two identical subunits of molecular mass (MM) 21.9 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The optical, electron paramagnetic resonance (EPR), and Mössbauer properties show that two of the iron atoms belong to FeS4 centers similar to Rd-type centers and the other two belong to an exchange-coupled binuclear center. Dfx is a monomer of molecular mass 14 kDa. The iron determination shows that this protein contains about two iron atoms per molecule. The optical, EPR, and Mössbauer properties show that these two iron sites are inequivalent.