Metallic nickel nanoparticles are excellent catalysts for the cracking of methane into carbon nanotubes and hydrogen. Decomposition of tetrahydrate nickel acetate proceeds readily in inert or hydrocarbon streams to produce metallic nickel agglomerates (average size of 10-80 nm) at around 350 degrees C. In the present study, this parent salt is thermally treated in methane streams in situ in a thermogravimetric analyzer (TGA), and weight changes corresponding to the carbon buildup in the metallic nickel particles are analyzed to provide some insight into the nickel-catalyzed cracking process. C to Ni atomic ratios (C/Ni) estimated directly from TGA data provided a systematic approach to study the catalytic activity of the nickel nanoparticles. Methane cracking starts at temperatures as low as 400 degrees C and continues efficiently until approximately 600 degrees C. Between 600 and 660 degrees C, methane decomposition momentarily breaks off, while presumably the catalytic system undergoes a self-reorganization. Cracking resumes at 660 degrees C and continues slowly up to 950 degrees C. The amount of carbon deposited in the 600-660 degrees C interval shows a linear dependence with methane concentrations, with C/Ni ratios ranging from 6 to 31. Transmission-electron microscopy images of the different C/Ni residues collected at 660 degrees C show, that during cracking, narrower carbon nanotubes are produced at elevated methane concentrations, suggesting dispersion of nickel nanoparticles. (C) 2008 American Vacuum Society.