Two different murdochite-type mixed oxides, (Mg-6-xLi(x))MnO8 (x = 0, 0.1, 0.2 and 0.3) and (Mg6-xAlx)MnO8 (x = 0, 0.2, 0.4, 0.6) were examined for the catalytic decomposition of N2O in order to make clear the effects of mixed valencies of pairing manganese ions and oxygen vacancies. The valence of manganese ions and the amount of surface oxygen vacancies have been examined with X-ray photoelectron spectroscopy (XPS). (Mg6-xLix)MnO8 had mixed valence manganese ions and oxygen vacancies on the surface after the substitution. The substituted (Mg-6-xAl(x))MnO8 had a mixed valence state but oxygen vacancies decresed with x and excess oxygen over stoichiometry was observed at x = 0.4 and 0.6. The reaction rate of N2O decomposition increased after substitution with lithium but hardly increased after the substitution with aluminum in (Mg-6-(x)A(x))MnO8. We assumed that the presence of oxygen vacancies on the surface along with pairing altervalent manganese ions affected strongly to enhance the reactivity of N2O decomposition.