Methanation reactions of CO or CO2 catalyzed by a commercial Ni/kiesel-guhr catalyst, and the adsorption of the relevant reactants and products, were studied with an in situ magnetic induction method using a low-field ac permeameter. At the conditions studied, CH4 was the only product observed and both reactions exhibited similar rates and trends. Comparison of the magnetic data under reaction to the individual contributions expected from adsorption suggests that adsorbed CO was the dominant surface species in CO methanation, while adsorbed CH4 was the dominant species in CO2 methanation. The reaction rate and relative magnetic loss were modeled using parameters obtained from single-species magnetic-adsorption data. Adsorptions were modeled with a Langmuir-type isotherm. Rates of methanation were successfully described by a Langmuir-Hinshelwood rate expression using the single-species adsorption parameters and extracting the surface-reaction rate constant. The magnetic loss was predicted then, using predetermined parameters, and compared to observations. Adequate prediction was observed for CO2 methanation whereas significant deviation exists for CO methanation. The latter result is due to the falsification of CO and possibly CH4 adsorption parameters by irreversible processes.