When the photoreduction experiment was performed in H2O using C2H2 (> 99.95%) as the feedstock, gas chromatography-mass spectrometry (GC-MS) revelated the formation of C2H4 (m/z = 28) as the only reaction product. This signal shifted to 32 when replacing H2O with D2O, which corresponds to the formation of C2D4 (m/z = 32) from C2D2 (m/z = 28), produced by the exchange between the feedstock C2H2 and D2O. As a result, we can conclude that the protons added to C2H2 to make the C2H4 product unequivocally originate from water.