The thermal stability of nanoscale grains in cryomilled aluminum powders containing 1% diamantane was investigated. Diamantane is a diamondoid molecule consisting of 14 carbon atoms in a diamond cubic structure that is terminated by hydrogen atoms. The nanostructures of the resulting cryomilled powders were characterized using both transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The average grain size was found to be on the order of 22 nm, a value similar to that obtained for cryomilled Al without diamantane. To determine thermal stability, the powders were heated in an inert gas atmosphere at constant temperatures between 423 and 773 K (0.51T(m) to 0.83T(m)) for exposure times of up to 10 h. The average grain size for all powders containing diamantane was observed to remain in the nanocrystalline range (1-100 nm) for all exposures and was generally less than half of that for cryomilled pure Al heated under the same conditions. The thermal stability data were found to be consistent with a grain growth model based on drag forces exerted by dispersed particles against grain boundary migration. The present findings indicate that the presence of diamantane results in a substantial increase in the thermal stability of nanoscale grains in Al.