Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine the adsorption and electropolymerization of 3-methylaniline (3-MA) and 3-ethylaniline (3-EA) on Au(111) single-crystal electrode in 0.5 M H(2)SO(4) + 6 mM 3-MA and 3-EA, respectively. 3-MA admolecules were adsorbed in a Au(111) (5 x 2 root 3)rect structure (theta = 0.2) at 0.5 V (vs reversible hydrogen electrode, RHE), but rearranged into two less compact adlattices, (5 x 2 root 3)rect, theta = 0.10 and (3 root 3 x 2 root 3), theta = 0.11, when the potential was increased from 0.5 to 0.8 V. In comparison, 3-EA admolecules were adsorbed in Au(111) (4 x 2 root 3)rect, theta = 0.125 at the onset potential (0.9 V) for electropolymerization. Raising the potential in excess of 0.9 V resulted in oxidation and polymerization of 3-MA and 3-EA molecules. The poly(3-MA) molecules produced in the early stage (<1 monolayer) assumed linear conformation, but became predominantly crooked upon the increase of overpotential. In contrast, poly(3-EA) molecules were mostly straight and aligned preferentially in the < 121 > directions of the Au(111) electrode. Linear poly(3-EA) chains continued to thrive with time and stacked up to produce a smooth polymeric film. Possible reasons for this sharp contrast in molecular conformation are discussed.