A series of [Ru-2(CO)(5)(mu-SCH2CH2CH2S)PXS] complexes (Ru-2-S-2-PXS, X = phosphine ligands, S = 1-8) have been synthesized and evaluated for their photocatalytic H-2 generation efficiencies from formic acid decomposition. The [Ru-2(CO)(5)(mu-SCH2CH2CH2S)P(o-C6H4CH3)(3)] (Ru-2-S-2-PX4) catalyst + P(CH3)(3) ligand exhibited a high turnover frequency of 15,840h(-1) and turnover number of 24,536. A mechanistic investigation of the Ru-2-S-2-PX4 + FA/TEA catalyzed photocatalytic H-2 generation reaction using ATR-IR, EI-MS, and NMR techniques suggested that when Ru2-S2-PX4 was photoirradiated, the P(o-C6H4CH3)(3) was dissociated from the complex to form a new species, [Ru-2(CO)(5)(mu-SCH2CH2CH2S)]* (I). The free P(o-C6H4CH3)(3) then attacks a second molecule of Ru-2-S-2-PX4 to form Ru-2-S-2-(PX4)(2) and release of free CO, which is then combined with species I to form Ru-2-S-2. Subsequent attachment of formate ion to species Ru-2-S-2-PX4, Ru-2-S-2, and Ru-2-S-2-(PX4)(2) to form [Ru-2(CO)(5)(mu-SCH(2)CH(2)CH(2)SA]-HC00(-) (II), Ru-2-S-2-HCOO- (II') and Ru-2-S-2-PX4-HCOO- (II"), respectively. Rearrangement of complex II (or II' or II") and evolution of CO2 generate a transient complex [Ru-2(CO)(5)(mu-SCH2CH2CH2S)H (III), which then undergoes a protonation process to yield complex [Ru-2(CO)(5)(mu-SCH2CH2CH2S)H-2] (IV). Release of H-2 and re-incorporation of the formate anion as well as evolution of CO2 regenerates the active complex III and the cycle begins again. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.