Methane and higher hydrocarbons were directly decomposed to hydrogen and carbon over molten magnesium. Hydrogen and micrometer size carbon particles were formed as products. The catalyst losses activity because of the evaporation of metal Mg. The activity of the catalyst can be recovered by heating the upper cold section of the reactor to slide Mg back to the reactor bed. Mg2C3 was identified as reaction intermediate in the reaction. Compared to the reforming process, the hydrocarbon direct decomposition process produces CO-free hydrogen, does not emit CO2 to environment, generates useful carbon powder, and is also an energy-saving approach for hydrogen preparation. To form each molar of hydrogen by methane direct decomposition only consumes about 65.1 % of the energy as that needed in the steam reforming of methane. The reaction is also proved to be useful in the decomposition of waste polyolefins, such as ploy olefin plastic and rubber. (c) 2007 Elsevier B.V. All rights reserved.