The surface reactions during the atomic layer deposition (ALD) of Ru using an organometallic precursor [CpRu(CO)2Et] and O2 gas were studied using quadrupole mass spectrometry. The analysis of the gas phase species showed that the surface chemisorption of CpRu(CO)2Et resulted in the formation of combustion products such as CO2, CO, and H2O. Additionally, H2 was detected as a major surface reaction product during the metal pulse. Strikingly, during the O2 pulse virtually no H2O or other H-containing reaction products were measured. The key conclusion of these observations is that dehydrogenation reactions occur on the Ru surface, resulting in the release of a large fraction of the precursor’s hydrogen, predominantly in the form of H2. On the basis of the catalysis literature, we infer that dehydrogenation reactions can also occur during other noble metal ALD processes.