Detailed studies on hydrogen evolution by decamethylruthenocene ([Cp* 2 Ru II ]) highlighted that metallocenes are capable of photoreducing hydrogen without the need for an additionals ensitizer.E lectrochemical, gas chromatographic, and spectroscopic (UV/Vis, 1 Ha nd 13 CNMR) measurements corroborated by DFTc alculations indicated that the production of hydrogen occurs by at wo-stepp rocess. First, decamethylruthenocene hydride [Cp* 2 Ru IV (H)] + is formed in the presence of an organic acid. Subsequently, [Cp* 2 Ru IV (H)] + is reversibly reduced in ah eterolytic reaction with one-photon excitation leadingt oafirst releaseo fh y-drogen. Thereafter,t he resultant decamethylruthenocenium ion [Cp* 2 Ru III ] + is further reduced with as econd releaseo f hydrogenb yd eprotonation of am ethyl group of [Cp* 2 Ru III ] + .E xperimental and computational data show spontaneous conversion of [Cp* 2 Ru II ]t o[ Cp* 2 Ru IV (H)] + in the presence of protons. Calculations highlight that the first reductioni se ndergonic (DG 0 = 108 kJ mol À1 )a nd needs an input of energy by light for the reactiont oo ccur.T he hydricityo ft he methylp rotons of [Cp* 2 Ru II ]w as also considered.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.