As shown in Table , this HER-enhancing effect can be generalized and was observed when the solvent was varied from the protic polar methanol to the aprotic polar acetone (Figure and Table , Entry 2). Variation of the pH from pH 6 to pH 4 does not affect the general trends observed (see Table , Entry 3 and Figure ). However, when using Et4N at pH 4, we still observe some HER activity (~ 40 % lower compared to the use of NH4 + , Figure ). In contrast, at pH 6, the HER evolution difference between Et4N + and NH4 + was much larger (Figure ). In the presence of Et4N + , HER-activity is reduced by ~ 96 % (based on TONs) compared with the NH4 + -containing system. This indicates that the observed effect is controlled by proton concentration in solution. To explore whether the observed HER enhancement by ammonium is linked to PS or CAT, we systematically varied both species. First, we replaced the original CAT, {Mo3} and used the literature-known cobaloxime HER CAT [Co(dmgH)2PyCl]. Here, we also observed drastically increased HER performance in the presence of ammonium ions, see Figure . This suggests that the HER-enhancing effect is not limited to one specific HER-CAT type and can be used for a range of different metal complex HER-CATs. In addition, the data suggest that the effect is most likely related to the PS-based reaction steps. To explore this hypothesis, we performed femtosecond (fs) and nanosecond (ns) transient absorption (TA) spectroscopy on Ru-PS in the presence of ammonium as well as tetraethyl ammonium ions (and the SED) to rationalize our observations from the catalytic experiments described above. Photoexcitation of Ru-PS in presence of the SED (ascorbic acid/ascorbate) led to the formation of reduced Ru-PS (i.e.