Kinetic and isotopic measurements for catalysts and conditions that rigorously excluded transport and thermodynamic artifacts led to a common sequence of elementary steps for reactions of CH4 with CO2 or H2O and for its stoichiometric decomposition on Ni/MgO catalysts. Turnover rates for forward reactions of CH4/CO2 and CH4/H2O mixtures were proportional to CH4 pressure (5-450 kPa) and independent of the partial pressure of the CO2 Or H2O coreactants (5-450 kPa). These turnover rates and their first-order rate constants and activation energies are also similar to those measured for CH4 decomposition, indicating that these reactions are mechanistically equivalent and that C-H bond activation is the sole kinetically relevant step in all three reactions. These conclusions were confirmed by identical CH4/CD4 kinetic isotope effects (k(H)/k(D) = 1.62-1.71) for reforming and decomposition reactions and by undetectable H2O/D2O isotopic effects. The kinetic relevance of C-H bond activation is consistent with the relative rates of chemical conversion and isotopic mixing in a CH4/CD4/CO2 mixture and with the isotopic evidence for the quasi-equilibrated nature of coreactant activation and H-2 and H2O desorption obtained from reactions of CH4/CO2/D-2 and (CH4)-C-12/(CO2)-C-12/(CO)-C-13 mixtures. These quasi-equilibrated steps lead to equilibrated water-gas-shift reactions during CH4 reforming, a finding confirmed by measurements of the effluent composition. These elementary steps provide also a predictive model for carbon filament growth and identify a rigorous dependence of the carbon thermodynamic activity on various kinetic and thermodynamic properties of elementary steps and on the prevalent concentrations of reactants and products, specifically given by P-CH4 P-CO/P-CO2 (or P-CH4 P-H2/P-H2O) ratios. These mechanistic features on Ni surfaces resemble those previously established for supported noble metal catalysts (Rh, Pt, Ir, Ru). These direct measurements of C-H bond activation turnover rates allowed the first direct and rigorous comparison of the reactivity of Ni and noble metal catalysts for CH4-reforming reactions, under conditions of strict kinetic control and relevant commercial practice and over a wide range of compositions and metal dispersions. (C) 2004 Elsevier Inc. All rights reserved.