Ordered arrays of similar to 4.5 nm diameter silver nanocrystals passivated with dodecanethiol show remarkable thermal stability at temperatures up to 975 K. Defects in the ordered-assembly were the first areas to show signs of melting while much of the structure remained intact. The mechanism for their stability is attributed to the unusually strong inter-nanocrystal bond formed by the interpenetration of thiol chains between neighboring nanocrystals within the nanocrystal superlattice. Individually dispersed nanocrystals show stability up to 800 K while some persist until 975 K.