The structural stability of nanocrystalline aluminum-manganese (Al-6.5 at.% Mn) alloys is studied in the temperature range of 200-400 A degrees C. Transmission electron microscopy shows that grain growth in this alloy is subdued by the presence of Mn, such that 100 nm or finer grain sizes can be retained at 200 and 300 A degrees C even after 1 month of annealing. In contrast, the principal mode of instability in the alloy is the precipitation of the equilibrium Al6Mn phase, which was observed to form at much shorter timescales and is present at 300 and 400 A degrees C after just 30 min. Differential scanning calorimetry was used to study the kinetics of the Al6Mn reaction using Johnson-Mehl-Avrami-Kolmogorov analysis and construct a time-temperature-transformation (TTT) diagram for this process. It is found that this Al-Mn single-phase nanostructured alloy can be stable against forming the Al6Mn phase and against grain growth for several months below 200 A degrees C and for short thermal excursions up to 300 A degrees C.