The initial atmospheric corrosion of zinc induced by formic acid, acetic acid, and propionic acid has been followed in situ with infrared reflection absorption spectroscopy (IRAS). An optical model was applied to quantify the IRAS data, which enabled the absolute amounts of the two main species detected, ZnO and zinc carboxylate (mainly hydrated or hydroxyl zinc carboxylate), to be derived. The formation rate of zinc carboxylate increases as propionic < acetic < formic acid and is mainly governed by the deposition velocity of each acid into the existing aqueous adlayer. The formation rate of ZnO and the overall corrosion rate of zinc increase in the order formic < acetic < propionic acid and are mainly attributed to the expected pH of the aqueous adlayer. The corrosion rates deduced are at least 1 order of magnitude higher than the experimentally determined field data due to a higher organic acid concentration used herein (120 ppb) and a higher average relative humidity (90%). (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3240878] All rights reserved.