The aim of this work is to quantitatively investigate the effect of La2O3 contents on the intrinsic activity of Ni active sites for the CO2 reforming of CH4 on Ni/La2O3 (1, 2, 4, and 6 wt%)/alpha-Al2O3 Catalysts using steady and pulse kinetic methods. The Ni particle sizes were decreased with increasing La,03 contents. And the 4-La catalyst had the most Ni-H species. The Ni active sites on the 4-La catalyst showed the highest reforming activity and the lowest activation energy. Among CH4, CO2, CO, and H-2, only CH4 reaction orders were not zero and they were affected by the La2O3 contents. In 550-600 degrees C, the CH4 dissociation is a rate-determining step for the reforming reaction on the Ni/La2O3/alpha-Al2O3 catalysts. The 4-La catalyst gave the highest reforming activity because the CH4 dissociation was a structure sensitive reaction and 4-La catalyst has the most suitable Ni particle size. In 650-750 degrees C, the CH4 dissociation and the formation of Ni-H reached equilibration and the reaction between CHx species with La2O2CO3 became the rate-determining step. The 4-La catalyst with the most Ni-H could keep the highest x value of CHx intermediates, leading to the highest reforming activity in 650-750 degrees C. Therefore, the La2O3 contents on the catalysts tuned the Ni particle sizes and Ni-H amounts on the catalysts, which lead to higher reforming activity on the 4-La catalyst. (C) 2007 Elsevier B.V. All rights reserved.