Mo dimer oxy-carbide species were synthesized in the supercage of NaY zeolite by adsorption of Mo(CO)(6) vapor at saturation, followed by thermal decomposition in vacuum. TPD, EXAFS,: XPS and XRF data revealed that a Mo dimer was attached in a supercage by a saturation adsorption-thermal decomposition at 573 K and that up to four Mo dimers in a supercage were stepwisely formed without aggregation after four cycles, The Mo dimer oxy-carbide species exhibited a unique methanol decomposition reaction to produce CH4, CO2, and H-2 in an approximate ratio of 1 : 1 : 2. This reaction was characteristic of the Mo dimer oxy-carbide species and was never observed fbr other Mo species such as oxidized and reduced Mo species in NaY supercages.