The conversion of solar energy into storable fuels, such as hydrogen, is a central concept to overcome diurnal and seasonal effects in solar radiation flux. Using sunlight to drive the splitting of water into oxygen and hydrogen is one of the most straight-forward reactions to this end. However, both halfreactions, i.e. the light-driven oxygen evolution reaction (OER) and the light-driven hydrogen evolution reaction (HER) are still formidable chemical challenges and require catalysts which are highly active, stable and economically viable. Molecular light-driven HER catalysis is a thriving field of research, as understanding the complex mechanisms and interactions which control HER activity can provide valuable insights for advanced component design. While early studies were focused on noble-metal (e.g. Pt) complexes as HER catalysts, recent research has explored earth-abundant metal complex HER-catalysts, e.g. based on iron, 9-12 cobalt or nickel as reaction sites. In addition, molybdenum sulfide complexes, or thiomolybdates, have recently received widespread attention as they show high HER activity under homogeneous light-driven and heterogeneous (photo-) electrocatalytic conditions. In addition, they are considered viable molecular models for amorphous molybdenum sulfide HER catalysts. Typically, homogeneous light-driven HER systems feature three key components: a photosensitizer (PS) for light-harvesting, charge-separation and electron-transfer, a catalyst (CAT) for the catalytic hydrogen evolution, and a sacrificial electron donor (SED) to provide reduction equivalents for re-generating the PS. The overall HER activity is not only controlled by the interplay of these species, but depends on the solvent, solution pH (or proton concentration, in organic solvents) and other factors, which are typically screened in initial catalytic optimization studies to identify ideal operating conditions. The impact of this complex set of experimental variables on the HER performance observed is often difficult to rationalize. This challenge can be illustrated by studies which explored the complex reactivity in a HER model system containing the sacrificial electron donor ascorbic acid/ascorbate, and model PS based on [Ru II (bpy)3] 2+ (bpy = 2,2'-bipyridine) or derivatives thereof. The following examples follow a common initial mechanism, where the photoexcited PS* is reductively quenched by the SED. Subsequently, the reduced PS˙ˉ transfers an electron to the CAT, see Figure . In this seemingly straight-forward set of reactions, unexpected effects of supposedly "innocent" (i.e., not participating in the reaction) spectator species have been described: In 2014 Scandola and colleagues demonstrated, that dehydroascorbic acid (DHA), which is the 2-proton/2-electron oxidation product of the SED ascorbic acid/ascorbate, inhibits light-driven HER catalysis due to highly efficient quenching of the reduced PS˙ˉ (i.e. [Ru(bpy)3] + ). In consequence, as HER catalysis proceeds, the DHA concentration increases and the reactivity-limiting effect of DHA becomes more dominant. Thus, deliberate decrease of the DHA concentration (by re-reduction to ascorbate) has been proven to result in increased light-driven HER performance. Intriguing pH effects were reported by Durrant and co-workers. The group investigated the pH optimum for the light-driven HER by the PS [Ru(bpy)3] 2+ and a Ni-phosphine catalyst. Photophysical and catalytic studies showed that the PS operates best at less acidic pH (due to a more efficient PS quenching by the SED ascorbate compared with ascorbic acid, Figure ), while the catalyst operates best at acidic pH, as protonation is required for efficient HER. Thus, an intermediate pH had to be chosen as compromise for optimum HER performance. Related studies also demonstrated that optimum catalytic HER performance is based on an intricate interplay between the PS:CAT:SED ratios, and theoretical prediction of the optimum performance ranges currently nearly impossible, as the underlying interactions between these species, and their effect on HER are often unknown. Here, we report an unexpected reactivity-enhancing effect of ammonium (NH4 + ) ions in light-driven, homogeneous HER catalysis. This effect is observed for a range of metal complex PS and metal complex CAT combinations in different solvents, suggesting that it is of a rather general nature. Preliminary mechanistic catalytic and photophysical studies provide initial insights into some of the underlying effects of the ammonium ions in one model reaction system. Initial experiments explored the catalytic performance of the light-driven HER in the system PS: [Ru(bpy)3] 2+ (= Ru-PS; 20 M); CAT: [Mo3S13] 2-(= {Mo3}; 0.3 M); SED: ascorbic acid/ascorbate (10 mM); solvent: MeOH:H2O (9:1, v:v), light source: LED (λmax = 470 nm, P ~ 40 mW/cm 2 ). These experiments showed major reactivity differences depending on the type of alkaline aqueous solution used to adjust the reaction pH (to pH 6). When an aqueous tetraethylammonium hydroxide (Et4NOH) solution was used for pH adjustment, very low hydrogen evolution was observed after 6 h irradiation (TON ~ 260, Fig. and Table ). In contrast, when aqueous ammonium hydroxide (NH4OH) solution was used for pH adjustment, we noted significant HER activity, reaching TONs of ~6,600 after tirradiation = 6 h (Figure and Table ), which is an increase by a factor of ~25. We could also show, that increasing concentrations of ammonium in the reaction medium (whilst maintaining the pH at pH =6, due to the ascorbic acid/ascorbate buffering effect) results in increasing HER activity (Figure ). This startling observation led us to study the use of other amine bases featuring N-H functions. As one example, we used triethylammonium hydroxide (Et3NHOH) for pH adjustment, and observed significant increase of HER-activity, see Table . Based on the available data, this preliminary study suggests that ammonium cations which feature one (Et3NH + ) or several (NH4 + ) N-H groups enhance the light-driven HER activity of the system studied.