Transient response isotopic tracing was used together with in situ infrared spectroscopy to elucidate the dynamics of several elementary processes believed to occur during CO hydrogenation over ruthenium catalysts. Chemisorbed CO was observed to exchange very rapidly with gas phase CO and under reaction conditions the two species are in equilibrium. A similar conclusion was reached regarding the relationship between gas phase H2 and adsorbed H atoms. The dissociation of molecularly adsorbed CO to form atomic carbon and oxygen was found to require vacant surface sites and to be reversible. It was shown that while CO is the principal adsorbed species present on the catalyst surface under reaction conditions, the catalyst also maintains a significant inventory of nonoxygenated carbon but no chemisorbed oxygen. It was also found that the rate at which nonoxygenated carbon undergoes hydrogenation is faster than the rate at which adsorbed CO is hydrogenated. This observation supports the hypothesis that the nonoxygenated carbon is an intermediate in CO hydrogenation.