Electrochemical reduction of cyclobutadienecyclopentadienylcobalt derivatives, C4Ra4-Co-C-5(HgY)(5) [R = C6H5- Y = -SMe (3), -SCOMe (4), -SCH2CH2SCH3 (5), -SCH2CH2SCOCH3 (6); R = p-EtOCOC6H4-, Y = -SEt (7), -SCOMe (8), -SCH2CH2SCH3 (9), -SCH2CH2SCOCH3 (10)3] and the parent C4R4-Co-C5H5 [R = C6H5- (11) and R = p-EtOCOC6H4- (12)] has been examined. Reversible one-electron "metal-centered" reduction of 11 (-2.24 V vs Ag/AgCl) yields a stable radical anion. In 12, it takes place at -1.70 V. In 3-10, similar metal-centered reduction occurs irreversibly, accompanied by an adsorption pre-wave at -0.7 to -1.15 V. We propose that the radical anion loses a -HgSX substituent soon after it is formed. Electrochemical cleavage of the substituents occurs before the metal-centered reduction in 4 and 8 and afterward in the others. The cleaved thiolate fragments yield an anodic peak. In all compounds with ester groups, eater reduction is observed at -2.1 to -2,5 V.