Abstract--Secondary minerals formed during simulated weathering of nuclear waste #asses have been identified by analytical electron microscopy. A complete description of the reacted #ass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk #ass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual #ass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the #ass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the #ass into crystalline phases did not occur at the interface with fresh #ass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment.