The intrinsic kinetic information for the dissociation of CH4 on the fresh Ni active sites was studied by pulsing CH4 through Ni/alpha-Al2O3 catalyst with different Ni particles. After the elimination of the mass and heat transfer, the conversions of CH4 were controlled far below the values of the thermodynamic equilibrium in order to keep the pulse reaction in the kinetic region. The coverage degree of carbon species on Ni active site is kept less than 1 to eliminate the influence of carbon deposition on the dissociation of CH4. The plot of TOFCH4 versus the Ni particle size exhibits a "mountain" shape. The Ea and In A for the CH4 dissociation are linearly increased with the increase in the Ni particle size, showing the compensation effect in the CH4 dissociation. This work reveals that the CH4 dissociation on fresh Ni active site of Ni particles in Ni/alpha-Al2O3 catalysts is a structure sensitive reaction. (c) 2006 Elsevier B.V. All rights reserved.