The system I cytochrome c maturation (Ccm) apparatus has been shown to handle a wide variety of apocytochrome substrates containing the CXnCH heme attachment sequence, where n = 2, 3, or 4 in natural sequences. When n = 5 or 6, the apparatus also appears to handle these substrates correctly, but close inspection reveals that the resulting mature cytochromes are mixtures of species containing extra mass. We have used accurate mass spectrometry to analyze peptide digests of matured Escherichia coil cytochrome cb(562) with n = 1, 5, or 6 and shown that an extra sulfur is sometimes incorporated into the heme-protein linkage. These unprecedented, aberrant persulfide linkages may shed new light upon the mechanism of the attachment of heme to substrate apocytochrome within the Ccm complex of E. coil.