The surface reaction products liberated during the atomic layer deposition (ALD) of Ru from (C5H5)Ru(CO)2(C2H5) and 18O2 were quantitatively analyzed using quadrupole mass spectrometry (QMS). The gas-phase reaction products during the Ru precursor pulse were CO2, CO, H2, and H2O, while during the O2 pulse primarily CO2 and CO were produced. Approximately 70% of the C atoms and 100% of the H atoms contained in the Ru precursor were released during the Ru pulse of the ALD process. From these observations, and on the basis of the surface science and catalysis literature, we conclude that the complex surface chemistry during Ru ALD can be described by catalytic combustion reactions. These reactions consist of the dissociative chemisorption of the Ru precursor’s ligands on the Ru surface during the metal pulse. These hydrocarbon ligands undergo dehydrogenation and combustion reactions on the catalytic metal surface in the presence of surface O formed due to the dissociation of O2 molecules in the previous O2 pulse. We postulate that the carbon-rich species produced due to the dehydrogenation of the Ru precursor’s hydrocarbon ligands self-limit precursor chemisorption by blocking adsorption sites. The use of the 18O2 isotope also unambiguously shows that a certain fraction of the precursor’s carbonyl ligands dissociate on the Ru surface.