The visible-light-driven HER activity of POM-PtI and POM-PtCl was examined using the photosensitizer [Ir(ppy)2(bpy)]PF6 (PS). PS was chosen due to its cationic charge, and the literature-known ability for light-driven electron transfer to POMs. In addition, (spectro-)electrochemical analyses of both POM-PtX species showed, that at least two electron transfers from the photoreduced PS (E1/2 ca. -1.8 V vs. Fc + /Fc) to POM-PtX is thermodynamically possible, as POM-PtX shows two redox-couples at potentials more positive than -1.8 V (see SI, Table , Figures ). Thus, light-driven catalytic studies were performed in water-free, de-aerated DMF solutions containing the respective catalyst POM-PtX (12.5 µM), the photosensitizer PS (125 M) and triethyl amine/acetic acid (TEA (1.0 M) / HAc (0.2 M)) as sacrificial proton/electron donors; this experimental setup has been adapted from earlier POM-based HER studies. The samples were irradiated under an Ar atmosphere using monochromatic LEDs (λmax = 470 nm, P ~ 40 mW cm -2 ) at 25°C. Hydrogen evolution was quantified by calibrated gas chromatography. Each data-point was recorded in triplicate, and averaged turnover numbers First, the HER activity of POM-PtCl, and the reference [Pt(bpy)Cl2] were studied. It was observed that under irradiation H2 evolution increases linearly with time for both catalysts, see Fig. . After tirradiation = 10 h, notable reactivity differences are observed. For POM-PtCl the TONs reached ~ 540, while for the reference [Pt(bpy)Cl2] only exhibits a TON of ca. 400.