Mo dimers were prepared in the supercage of NaY zeolite by cycles of saturated adsorption of Mo(CO)(6) at room temperature and subsequent thermal decomposition at 573 K. The obtained samples were characterized by temperature-programmed desorption, extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectra, and X-ray fluorescence. Two Mo(CO)(6) molecules per supercage were immobilized in each process. After the thermal decomposition of the adsorbed Mo(CO)(6) in the supercage, the resultant Mo(1)/NaY could accept two more Mo(CO)(6) molecules per supercage. Mo loading was found to increase linearly by two Mo(CO)(6) molecules in a supercage per one deposition cycle, and was saturated with eight Mo atoms per supercage after the four cycles. EXAFS analysis revealed that Mo(II) oxocarbide dimer species [Mo-2(C)(O)(x)] with the Mo-Mo distance of 0.285 nm was formed in the supercage by the 573 K thermal decomposition of the adsorbed Mo(CO)(6). The structures of Mo oxocarbide dimers were invariant regardless of the Mo loading.