Our transfer hydrogenation reaction of acetylene by taking TFE as hydrogen source is a promising alternative over conventional hydrogenation technology with a flammable H2 atmosphere. When the photoreduction was instead performed using deuterated TFE as deuterium source, we observed the formation of C2D4 (m/z = 32) from C2D2 (m/z = 28) (Figure ). C2D2 is produced by exchange between the feedstock C2H2 and deuterated TFE, which we pre-equilibrated before illumination. The addition of two deuterium to C2D2 yields C2D4. These two experiments prove that acetylene is the precursor for the observed C2H4 and that the protons added to make the C2H4 reduction product originate from TFE. Additionally, we show that water can be the source of protons for the reaction (Figure ), which is advantageous from a sustainability standpoint as it avoids the use of an organic, fossil-fuel derived proton source. species (Figure ). In order to detect the formation of paramagnetic Co(II) we used electron paramagnetic resonance (EPR) spectroscopy, which showed the EPR signal from vs. SCE in acetonitrile) to the Co II (Co II /Co I reduction potential of -1.13 V vs. SCE in acetonitrile) to form the low valent Co I species. We observed that the unimolecular quenching rate constant for BIH exceeds that of [Co(dmgH)2pyCl] by a factor of nearly 100 according to Stern-Volmer analysis (Figure ). As a result, we conclude that [Ru(bpy)3] 2+