Here, we study the stability of the 2D organization of thiol-coated silver nanoparticles (NPs) by transmission electron microscopy. Whatever the alkyl chain length and the nature of the silver precursor, we show the rapid corrosion (over a few days) of the NPs by O2 from laboratory air whereas they remain stable for several weeks under a nitrogen atmosphere. We show that this phenomenon is amplified by the humidity in the air and by thiols trapped in the NP monolayers. We obtain evidence of these thiols in excess by infrared and energy-dispersive spectroscopies. This study of stability has been extended to gold nanoparticles (AuNPs) coated with dodecanethiols. The AuNPs remain stable under laboratory air because of the higher redox potential of Au compared to that of Ag and O2.