Temperature-programmed decarbonylation processes of Mo(CO)(6) entrapped in NaY and HY zeolites were investigated in situ by the time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) technique. The DXAFS study revealed that the decarbonylation of Mo(CO)(6) in both NaY and HY proceeded through molybdenum subcarbonyl species and that the subsequent formation of decarbonylated species is affected by the stability of the subcarbonyl intermediate species. In the case of Mo(CO)(6)/NaY, stable Mo(CO)(3)(O-L)(3) (O-L, oxygen atoms of the zeolite framework) species were formed at 400-500 K and Mo(II) oxocarbide dimeric species Mo-2(C)O-x were formed above 500 K. In the case of Mo(CO)(6)/HY, on the other hand, unstable MO(CO)(3)(O-L)(x) (x = 1-2) species were formed around 450 K and they reacted easily with protons in zeolite supercages to be converted to Mo(II) monomer species. An unstable intermediate species was observed, and its structure was determined by DXAFS for the first time.