The mechanism of methanation over silica supported nickel was investigated using a novel isotope transient tracing of the reaction using (CO)-C-13. Tracing of the reaction path was performed in a rapidly recirculating gradientless reactor running at I arm and 210 degrees C. It is shown that a combined mechanism consisting of two parallel paths, where both paths lead to the production of methane from synthesis gas, describes the experimental data satisfactorily. Our results suggest that assuming catalyst sites with varying degree of adsorption strengths may properly characterize a catalyst and transient tracing can be successfully utilized to estimate the catalyst properties.