Two tetranuclear iron–sulfur complexes, (μ,μ-pbtt)[Fe2(CO)6]2 (pbtt = benzene-1,2,4,5-tetrathiolato, 3) and (μ,μ-obtt)[Fe2(CO)6]2 (obtt = benzene-1,2,3,4-tetrathiolato, 4), were prepared from reaction of Fe3(CO)12 and the corresponding tetramercaptobenzene in THF, respectively. Complexes 5 and 6, (μ,μ-pbtt)[Fe2(CO)5L1][Fe2(CO)5L2] (L1 = CO, L2 = PPyr3 (Pyr = N-pyrrolyl), 5; L1 = L2 = PPyr3, 6) were obtained by controlling CO displacement of 3 with PPyr3. Molecular structures of 3–6 were determined by spectroscopic and single-crystal X-ray analyses. All-CO Fe4S4 complexes 3 and 4 each display four-electron reduction processes in consecutive chemically reversible two-electron reduction events with relatively narrow potential spans in the cyclic voltammograms. Phosphine-substituted Fe4S4 complexes 5 and 6 exhibit two consecutive two-electron reduction events, which are not fully reversible. The electrocatalytic properties of 3 and 4 for proton reduction were studied using a series of carboxylic acids of increasing strength (CH3COOH, CH2ClCOOH, CHCl2COOH, CCl3COOH, and CF3COOH). The mechanisms for electrochemical proton reduction to hydrogen catalyzed by complex 3 as a function of acid strength are discussed