With growing concerns about global environmental and increased focus on population health is renewed interest in transportation sector. The challenge is to find and develop cost effective ways to improve urban air quality without scarifying economy. The natural gas, as fuel, constitutes an alternative to the traditional fuels because its impact on the reduction of the global warming impact is consequent compared to the traditional fuels such as gasoline. This study deals with a study of NGV's catalysts. Natural gas vehicles' catalysts were characterized by XPS, STEM in order to determine the various active phases. The results of characterization showed that the palladium oxidized Pd2+ was the principal active site. The experimental runs were carried out close to the real operating conditions using a sample of monolith. Numerous reactions were identified, such as NO reduction by hydrogen, carbon monoxide at low temperature and methane at high temperature. Moreover, methane reforming and water-gas shift were clearly found on commercial system. The influence of each reaction on the kinetics behaviour of the whole mixture makes difficult to uncouple the different reactions of oxidations and reductions, because the rate of these reactions was highly dependant on the reactant present in the automobile exhaust gases as well as their relative concentration. To identify each involved reaction in NGV TWC process, we remove alternatively reducing agents or oxidizing agents. (C) 2008 Elsevier B.V. All rights reserved.