The initial atmospheric corrosion of a brass (Cu-20Zn) alloy induced by carboxylic (formic, acetic, and propionic) acids has been investigated through surface or near-surface analytical techniques. The main reaction products identified were Zn-carboxylate and a Cu2O-like layer. Their formation kinetics reveal the galvanic effects due to Volta potential variations along the surface, which result in corrosion cells with central anodic formation of zinc-carboxylate and peripheral cathodic formation of Cu2O. The corrosion cell size increases as propionic < acetic < formic acid, reflecting the dependence on ionic conductivity of the aqueous adlayer. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3577600] All rights reserved.