Oxidation of aniline and its derivatives can yield cationic radicals, which induce chain reactions to produce polymeric materials with notable conductivity. When these events occur on ordered electrode surface, admolecules can be adsorbed in some specific manner that guides reactions to proceed in some specific directions to yield products with well-defined conformations. This study employed cyclic voltammetry, X-ray photoemission spectroscopy (XPS), and in situ scanning tunneling microscopy (STM) to study the adsorption and polymerization of metanilic acid (MA) in 0.5 M H2SO4. MA molecules were adsorbed in highly ordered adlattices characterized as Au(1 1 1) - (root 19 x root 31) and (2 root 7 x root 31) at 0.5 and 0.8 V (vs. reversible hydrogen electrode). These adlattices were displaced by bisulfate anions at E > 1.0 V. MA molecules still could be oxidized to yield polymer molecules, which lacked a well-defined structure. By contrast, MA and aniline molecules could be coadsorbed in a highly ordered Au(1 1 1) - (4 x 2 root 3) structure at 0.8V in 0.5M H2SO4 + 30 mM aniline + 3 mM MA, which led to anisotropic oxidative polymerization in the < 1 2 1 > directions of the Au(1 1 1) electrode surface. According to ex situ XPS results, the as-produced linear polymeric chains were mainly polyaniline (PAN), rather than copolymer of aniline and MA. The surface structure and polymerization of aniline and MA varied greatly with their molar ratios. No ordered adlayer was observed in solution containing equal amounts (30 mM) of MA and aniline. (C) 2013 Elsevier B.V. All rights reserved.