The electron microprobe analyzer has been applied to a study of chemical reactions and diffusion in a number of nuclear fuel materials. During manufacture by powder metallurgy techniques, UN fuel particles underwent complete chemical reaction with the aluminum matrix in which they were embedded, while U3 Si and UC fuel particles showed partial chemical reaction. Subsequent heating completed the U3 Si-Al and UC-Al reactions. Diffusion was observed to have taken place across the boundaries in UO2-ZrO2 couples after exposure to elevated temperatures.