SSITKA coupled with transient isothermal (TIH) and temperature-programmed hydrogenation (TPH) were used to investigate the influence of Co particle size (d(Co) = 11-21 nm) and time-on-stream, TOS (up to 25 h) on important kinetic parameters of CO methanation at 230 degrees C on Co/gamma-Al2O3. TOFCO and TOFCH4 were independent of d(Co) and decreased with TOS. The theta(CO) and theta(CHx) (active CO-s and CHx-s) slightly increased with d(Co) as opposed to theta(CxHy) (inactive species), which remained practically constant. On the contrary, theta(CHx) and theta(CxHy) largely decreased and increased, respectively, while theta(CO) stayed practically constant with TOS. An inverse D-KIE independent of d(Co) was estimated by a novel (CO)-C-13/D-2-SSITKA experiment, reported for the first time to the best of our knowledge. The reason of catalyst deactivation was the decrease of k(CHx) (hydrogenation of -CHx) likely because of the influence of -CxHy on the bonding of active species and on theta(H) (a decrease was determined). (c) 2020 Elsevier Inc. All rights reserved.