We report the effect of temperature on the stability of glutathione-protected Ag25 clusters. The clusters are stable up to 50 C. Interestingly, above this temperature, they decompose to yield Ag 2S nanoparticles with an average diameter of 3 ± 1 nm, crystallizing in monoclinic acanthite polymorph. Unlike conventional methods of syntheses of Ag2S, where a temperature of ∼200 C is needed, our study shows the possibility of synthesis of Ag2S nanoparticles at much lower temperatures. This is in contrast with silver nanoparticles protected with thiolates, which typically give silver and alkyl/aryl disulfide upon thermal activation. The mechanism of cluster decomposition and formation of silver sulphide nanoparticles was investigated using various analytical techniques such as ultraviolet-visible spectroscopy, X-ray diffraction scanning electron microscopy, energy-dispersive analysis of X-rays, transmission electron microscopy, and electrospray ionization mass spectrometry. The monolayer of the cluster undergoes S-C bond cleavage, as revealed by mass spectrometry. This is somewhat unusual because Ag-S cleavage is expected in view of its lower bond energy. © 2012 American Chemical Society.