The mechanism of methane activation in CH4-CO2 reaction over various unsupported and supported Mo2C catalysts was studied by means of an isotopic tracer technique employing (CH4)-C-13 and CD4 as tracers. incorporation of carbide carbons into CO took place in the case of CH4CO2 reaction over unsupported Mo2C, which was retarded significantly by dispersing Mo2C onto ZrO2 support. Methane dissociation step was also retarded significantly over supported Mo2C catalysts, and the durability of the catalysts was improved by preventing inactive carbon accumulation. By dispersing Mo2C on Al2O3, novel active acidic sites were produced, where CH4-CD4 exchange reaction took place even below room temperature through an associative mechanism. (c) 2005 Elsevier B.V. All rights reserved.