[en] The results of this study are in two major parts. In Part I the use of steady-state isotopic transients of multiple elements (C, H, and O) under actual methanation reaction conditions has permitted an assessment of the reactivity of water on a Ni powder catalyst. It was concluded based on the addition of isotopic water that oxygen, once reacted to form water, is able to readsorb even where the surface coverage of CO remains high. At the low relative partial pressures of water used, however, there was no effect of added water on the formation of methane. The surface residence time of water determined from isotopic transients contains the residence time on the surface during the primary formation reaction as well as the time spent during readsorption(s). Part II addressed how a catalyst modifier (in this case Cl) affects methanation in CO hydrogenation using steady-state isotopic transient kinetic analysis (SSITKA) of methanation. The results obtained using silica-supported Ru suggest the structural rearrangements induced by the presence of chlorine, rather than selective site blocking or electronic interactions, may be the primary mechanism of chlorine modification of the catalytic properties of supported metals for CO hydrogenation. SSITKA indicated that the decrease in methanation activity with increasing initial Cl concentration was a function of a decrease in the number of reactive surface intermediates (or sites) and not of a change in site activity. 36 refs., 10 figs., 5 tabs