Silica-supported molybdenum oxide (MoO3/SiO2) and alkali-ion-modified MoO3/SiO2 samples were characterized by Mo K-edge XAFS and Raman spectroscopies. The structure of the molybdenum species in MoO3/ SiO2 was confirmed to be monomeric and tetrahedral, (O=Mo=O)O-2. In alkali-ion-modified MoO3/SiO2, the structure of the molybdenum species varied with the kinds and/or the amounts of added alkali ions. In Na+-modified MoO3/SiO2 with sodium ions of the molar ratio Na/Mo < 1.0, the molybdenum species were mainly Na2Mo2O7 besides (O=Mo=O)O-2 which was present in MoO3/SiO2, and the addition of sodium ions of molar ratio Na/Mo > 1.0 caused the formation of Na2MoO4 at the expense of Na2Mo2O7. In the case of K+- and Rb+-modified MoO3/SiO2, the addition of alkali ions equimolar with the molybdenum atoms in the samples brought about the formation of Mo2O7 species, and by the addition of alkali ions of molar ratio (alkali ion/Mo) > 1.0, MoO4 species was formed on the silica support. In the molybdenum species in K-MS and Rb-MS with alkali ions of molar ratio (alkali ion/Mo) < 1.0, the proportion of an octahedral MoO6 unit to all the molybdenum species in the samples was higher than that in K-MS and Rb-MS with alkali ions of the molar ratio > 1.0.