The corrosion mechanism of copper at 373 and 300 K in the presence of submicron (NH4)2SO4 particle deposits has been investigated. Several in situ techniques have been used to monitor the corrosion process in real time. At and above the critical relative humidity of (NH4)2SO4, dissolution of Cu is followed by formation of Cu2O, oxidation of Cu(I) ion to Cu(II) ions, and precipitation of antlerite [Cu3(SO4)(OH)4], bronchantite [Cu4(SO4)(OH)6], or posnjakite [Cu4(SO4)(OH)6. H2O]. The amount of corrosion product formed increases with amount of (NH4)2SO4 particles, relative humidity (RH), and temperature. The in situ techniques allowed us to confirm and refine the individual steps in the multistep mechanism proposed in earlier work.