[Ru(bpy)3] + ), as detected in measurements by the characteristic absorption at 510 nm (ESI, Figure ). This longlived feature decays on a µs -ms timescale, as probed by ns-TA. These studies revealed changes in the photophysical behaviour of Ru-PS, depending on the presence or absence of NH4 + , see Figure . Specifically, the lifetime of the reduced Ru-PS˙ˉ approximately doubles in the presence of ammonium ions (10 mM). In contrast to earlier reports on Ru-PS˙ˉ in a DMF/sodium ascorbate system, we observed a bi-exponential decay, in the presence of ammonium or tetraethyl ammonium (Figure ). This observation is tentatively assigned to the presence of oxidised SED, which is known to act as an efficient quencher for the reduced Ru-PS˙ˉ. 20 this could lead to two independent decay pathways, i.e. a geminate-and nongeminate-like decay. Analysis of the fs kinetics, which represent the reductive quenching of the photoexcited Ru-PS, indicates no observable effect of NH4 + or EtN4 + (Figure ). This suggests that the formation of the reduced Ru-PS˙ˉ is not affected by the presence these ions. Nonetheless, the increased lifetime of the reduced photosensitizer is generally considered beneficial for HER catalysis and could be one major contributor to the observed HER-activity increase. In addition, the lifetime of the reduced Ru-PS˙ˉ increases with increasing ammonium concentration, emphasizing that the concentration-dependent effect of ammonium on the HER activity (Figure ) could be due to increased lifetimes of the Ru-PS under the catalytic conditions studied. To the best of our knowledge, this behaviour has not been reported previously. To assess whether this reactivity is specific to [Ru(bpy)3] 2+ , we performed additional catalytic experiments under the conditions described above, where the original Ru-PS was exchanged for structurally related Ru-or Ir-based photosensitizers. As shown in Table , in all cases studied, the HER-enhancing activity of ammonium was observed, and increase of H2 evolution (under otherwise identical reaction conditions) by factors up to >30 was observed. Finally, we hypothesized that the ammonium ions might be interacting with the SED (ascorbic acid/ascorbate), as electrostatic as well as hydrogen bonding interactions between both species are possible. To explore whether the observed HER-enhancement is specific for ascorbic acid/ascorbate, we performed a reference experiment where N-benzyl-1,4dihydronicotineamide (BNAH) is used as alternative SED (using Ru-PS and {Mo3} as PS and CAT, respectively), see Table . This study also showed increased light-driven HER activity, however, here, the observed increase in H2 evolution was only a factor ~ 2.5. In sum, we report the surprising HER-enhancing effect of NH4 + cations when used with a range of metal complex photosensitizers and metal-based molecular HER catalysts. Initial studies show that the enhancing effect is not limited to a specific type of photosensitizer, catalyst or electron donor, and preliminary photophysical studies indicate that changes to the photophysical properties of the model PS studied might be one of the reasons for the observed reactivity increase. Other possible mechanisms which could contribute significantly to the observed reactivity enhancement are electrostatic and hydrogen bonding effects, aggregation/ion pairing as well as proton-management by the ammonium ions. Subsequent studies will build on this initial explorative study and use insitu/operando experimental methods including time-resolved spectroscopies and spectro-electrochemistry together with computational analyses to rationalize the observed reactivity enhancement and provide in-depth understanding of which systems benefit most from the reactivity increase.