Irradiation at 460 nm of [Mo-3(mu(3)-S)(mu(2)-S-2)(3)(S2CNR2)(3)]I ([2a]I, R = Me; [2b]I, R = Et; [2c]I, R = iBu; [2d]I, R = CH2C6H5) in a mixed aqueous-polar organic medium with [Ru(bipy)(3)](2+) as photosensitizer and Et3N as electron donor leads to H-2 evolution. Maximum activity (300 turnovers, 3 h) is found with R = Bu-i in 1:9 H2O:MeCN; diminished activity is attributed to deterioration of [Ru(bipy)(3)](2+). Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo-3(mu(3)-S)(mu(2)-S-2)(3)(S2CNR2)(3)](+) to [Mo-3(mu(3)-S)(mu(2)-S)(3)(S2CNR2)(3)](+) via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [Mo3S6(S2CNR2)(3)](+) or [Mo3S5(S2CNR2)(3)](+) species. Deliberate preparation of [Mo3S4(S2CNEt2)(3)](+) ([3](+)) and treatment with Et2NCS21- yields [Mo3S4(S2CNEt2)(4)] (4), where the fourth dithiocarbamate ligand bridges one edge of the Mo-3 triangle. Photolysis of 4 leads to H-2 evolution but at similar to 25% the level observed for [Mo3S7(S2CNEt2)(3)](+). Early time monitoring of the photolyses shows that [Mo3S4(S2CNEt2)(4)] evolves H-2 immediately and at constant rate, while [Mo3S7(S2CNEt2)(3)](+) shows a distinctive incubation prior to a more rapid H-2 evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [Mo3S7(S2CNiBu2)(3)](+) left undisturbed over 24 h deposit the asymmetric Mo6 cluster [((Bu2NCS2)-Bu-i)(3)(mu(2)-S-2)(2)(mu(3)-S)Mo-3](mu(3)-S)(mu(3)-eta(2),eta(1)-S',eta(1)-S ''-S-2)[Mo-3(mu(2)-S)(3)(mu(3)-S)((S2CNBu2)-Bu-i)(2)(mu(2)-(S2CNBu2)-Bu-i)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H-2-evolving catalysts in photolyses beginning with [Mo3S7(S2CNR2)(3)](+). When used as solvent, N,N-dimethylformamide (DMF) suppresses H-2-evolution but to a greater degree for [Mo3S4(S2CNEt2)(4)] than for [Mo3S7(S2CNEt2)(3)](+). Recrystallization of [Mo3S4(S2CNEt2)(4)] from DMF affords [Mo3S4(S2CNEt2)(4)(eta(1),kappa O-DMF)] (5), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H-2 evolution. Computational assessment of [Mo3S4(S2CNMe2)(3)](+) following addition of 2H(+) and 2e(-) suggests a Mo(H)-mu(2)(SH) intermediate as the lowest energy species for H-2 elimination. An analogous pathway may be available to the Mo-6 cluster via dissociation of one end of the mu(2)-S2CNR2 ligand, a known hemilabile ligand type, in the [Mo3S4](4+) fragment.