Europium amide species prepared by dissolving Eu in a liquid ammonia were supported on alumina, and the samples (Eu/Al2O3) were evacuated at various temperatures. The samples exhibited the activity for the isomerization of 2,3-dimethylbut-1-ene to 2,3-dimethylbut-2-ene, and the catalytic activity changed remarkably with the evacuation temperature and the loading amount of Eu. X-ray absorption near edge structure (XANES) spectra of the low loading (4 wt.%) and high loading (8 wt.%) Eu. Eu/Al2O3 showed that Eu species in these samples are the mixture of Eu2+ and Eu3+ ions and the composition of di- and trivalent ions changes with the evacuation temperature. Structural analyses by extended X-ray absorption fine structure (EXAFS) spectra indicated that Ea ions preferentially react with OH groups on an Al2O3, surface to form the isolated Eu species in 4 wt.% Eu/Al2O3, while the additional Eu species consisting of the Eu atoms bridged by the nitrogen atom are formed in 8 wt.% Eu/Al2O3. It has been revealed that the former Eu species is inactive for the isomerization of 2,3-dimethylbut-1-ene, but the latter is active. These results were in good agreement with those obtained from IR measurements. EXAFS analysis also clarified close relationship between the catalytic activity and the degree of the aggregation of the active Eu species.