The use of nanocrystalline (nc) alloys at elevated temperatures has been severely limited by their inherent thermal instability.  In order to better understand the effect of nanoscale particles on this behavior, grain growth in nanocrystalline Al alloys with 1 and 5 wt% diamantane diamondoid additions was investigated.  Diamantane diamondoids are hydrocarbon molecules with a 14 C atom diamond cubic framework terminated by hydrogen atoms (C14H20). In the present work, as-cryomilled powders were sealed in glass tubes in an Ar atmosphere and then annealed at temperatures ranging from 423–773K for different hold times up to ten hours.  The average grain size for nc Al + 1 wt% diamantane was limited to 55 nm or lees from an initial grain size of 22 nm for all of the exposures conducted.  Similar results were obtained with nc Al + 5 wt% diamantane.  The thermal exposure data also demonstrate that the average grain size following thermal exposure for nc Al with diamantane is substantially smaller than that for pure nc Al processed under the same conditions.  Analysis of the grain growth data suggests that the presence of diamantine results in strong pinning forces on boundaries during heat treatment.  The present work was initially supported by the National Science Foundation (Grant No. D-DMR-0304629), and is currently sponsored by the UC Discovery Program (Award No. GCP07-10250) in partnership with the Boeing Company (Award No. 247099).