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 glass, 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.