Finally, we were interested in understanding the causes of the loss of HER activity observed after ~ 9 h of irradiation (Figure ). We hypothesized (based on initial UV-Vis spectroscopy of the reaction solution, SI, Figures ), that this could be due to PS degradation, and thus, loss of the light-harvesting capabilities of the system. To experimentally probe this, we performed a reusability experiment where the standard photochemical experiment (identical to the experiment shown in Figure , catalyst: POM-PtI) was performed. After 10 h of irradiation, an aliquot of fresh PS (110 M) was added to the reaction mixture and irradiation was resumed. As illustrated in Figure , the system resumed hydrogen evolution at high reactivity. This observation provides evidence that PS degradation is a major factor in reactivity loss in the system described, while catalytic activity by the POM-PtI is still observed. Thus, future studies can explore the design of more robust photosensitizers with optimized POM binding behavior. 66 To gain further insight into the stability of POM-PtX, we performed a range of mechanistic analyses: First, UV-Vis spectroscopy of POM-PtX in the reaction solvent DMF showed no spectral changes over 18 h (SI, Figure ). Also, microfiltration and UV-Vis spectroscopic analysis of a reaction mixture containing POM-PtI and PS (molar ratio: 1/10) showed no spectral changes, indicating that no colloidal particles are observed (SI, Figure ). This was supported by dynamic light scattering (DLS) on the catalytic reaction solution, where no particles within the measurement range (1 -10000 nm) were detected. This suggests that the system does not produce colloidal platinum particles as the active HER catalyst, but that a molecular system operates as HER catalyst in the present study. Note that the "classical" mercury amalgam test for Pt colloids was not suitable for the present system, as elemental Hg undergoes redox-reactions with POM-PtX already in the dark, resulting in fast degradation of the POM-PtX molecule. Similar behavior has been reported before. To provide further evidence for the molecular nature of the HER system reported, we performed a catalytic experiment under the standard conditions described in Figure using POM-PtCl as model catalyst. Then, excess nBu4NI was added to the reaction solution to trigger iodide-for chloride ligand exchange on POM-PtCl (leading to partial in-situ formation of POM-PtI). As predicted from our initial experiments (Figure ), this led to an increase of the TON (+ ~ 20 %), thereby providing further evidence of a molecular HER catalysis (SI, Figure ). Conclusion