Abstract: EELS based on features of ELNES of M4 and M5 ed ges can provide information regarding oxidation states of Ce in proposed host ph ases of radioactive wastes (e.g., pyrochlore) and other REE- bearing minerals. M4 and M5 edges of Ce4+ have higher energy-loss than those of Ce3+. Intensity ratio between M4 and M5 edges and weak peaks after the M4 and M5 edges can be used for quantifica tion of Ce3+ and Ce4+. EELS data from an unaltered area of pyrochlo refrom Inner Mongolia o f China indicate Ce3+ and other rare earth elements (REE). However, EELS spectrum from the neighboring altered area shows Ce4+, Ba and other REE. The oxidation of Ce results in loss of REE, U, and alpha decay product Pb during the alteration of the pyrochlore. Based on features of ELNES of L3 edges, EELS spectra can also provide quantitative information regarding oxidati on states of Fe in Fe-bearing amphiboles, Fe-bearing pyroxenes, and other sili ca te minerals. There is a large difference in Fe oxidation states of the amphibole between the directly measured and indirectly calculated methods.