The combination of matrix isolation infrared spectroscopic and quantum chemical calculation results indicate that the NbO 2 molecule is coordinated by two noble gas atoms in forming the NbO 2 (Ng) 2 (Ng=Ar, Xe) complexes in solid noble gas matrixes. In contrast, the TaO 2 molecule is not able to form similar noble gas complex. The niobium and tantalum dioxides further react with dioxygen to form the side-on bonded superoxo-dioxide complexes MO 4 (M=Nb, Ta), which are coordinated by one argon atom in solid argon matrix. The coordinated Ar atom in MO 4 (Ar) can be replaced by O 2 or Xe in forming the MO 6 and MO 4 (Xe) complexes. The results indicate that the NbO 2 , NbO 4 and TaO 4 molecules trapped in solid noble gas matrixes should be regarded as the NbO 2 (Ng) 2 and MO 4 (Ng) (Ng=Ar, Xe; M=Nb, Ta) complexes instead of “isolated” metal oxide species.