The catalytic oxidation of CO by O2 to form CO2 over Pt based catalysis is one of the most studied catalytic reaction leading to controversial mechanism descriptions. Steady State Isotopic Transient Kinetic Analysis (SSITKA) coupled with InfraRed (IR) spectroscopy is a powerful technique to study heterogeneous reaction mechanisms combining both the observation of adsorbed species on the catalyst surface and kinetic measurements. In this paper, SSITKA-IR technique was applied to study the CO oxidation reaction in presence and absence of CO2 in order to distinguish between active and spectator intermediates formed during the reaction. Linear carbonyl and bridged carbonyl type species adsorbed on metallic Pt0 were clearly identified as active intermediates at low temperature (131 °C). In contrast, the hydrogen-carbonate species formed during CO oxidation reaction were proven to be inactive species in CO2 formation but rather due to the re-adsorption of CO2 product itself on alumina surface.