The reduction of Fe-2(mu-SC6H4S)(CO)(6) (1) at glassy carbon electrode appears chemically reversible (E-1/2 = -1.44 V vs. Fc in Bu4NPF6-CH2Cl2, k(s,h) similar to 5 x 10(-3) cm s(-1)) and actually consists of two overlapping one-electron transfers with the second transfer slightly more favorable than the first (E-2(0) - E-1(0) similar to 35 mV). Apparently, the reduction of Fe-2(mu-SCH2C6H4CH2S)(CO)(6) (2) is also a two-electron process (E-1/2 = 1.68 V vs. Fc in Bu4NPF6-CH2Cl2), but 2(2-) decomposes on the voltarnmetric time scale. For both 1 and 2, the electrochemical reduction of the neutral cluster generates intermediate species which further catalyze proton reduction in acid-containing electrolyte (i.e., HBF4 in Bu4NPF6-CH2Cl2). Unfortunately voltammetry also reveals a catalytic cycle for electrochemical proton reduction either slow or complicated by side reactions. (C) 2003 Elsevier B.V. All rights reserved.