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We report a very efficient homogeneous system for the visible-light-driven hydrogen production in pure aqueous solution at room temperature. This comprises [RhIII(dmbpy)2Cl2]Cl ( 1) as catalyst, [Ru(bpy)3]Cl2 ( PS1) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1/ 1/ascorbate/ascorbic acid system is by far the most active rhodium-based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na3[RhI(dpm)3Cl] and [RhIII(bpy)Cp*(H2O)]SO4 and 2) the system is less efficient when [IrIII(ppy)2(bpy)]Cl ( PS2) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium-based H2-evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1/ 1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h−1. Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H2-evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1, which is then able to reduce [RhIII(dmbpy)2Cl2]+ to [RhI(dmbpy)2]+. This reduced species can react with protons to yield the hydride [RhIII(H)(dmbpy)2(H2O)]2+, which is the key intermediate for the H2 production. [ABSTRACT FROM AUTHOR]