Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were employed to study the adsorption and polymerization of the geometric isomers of ethylaniline (EA) on a Au(111) single-crystal electrode in 0.5 M H2SO4. All three isomers, namely o-, m-, and p-EA, were adsorbed in highly ordered structures, identified as Au(111)-(4 × 2√3)rect for m- and p-EA and (4√3 × 4√3)R30° for o-EA, at the onset potentials (∼0.9 V [vs. reversible hydrogen electrode]) for electropolymerization. Raising the potential in excess of 0.9 V resulted in oxidation and polymerization of m- and o-EA, but decomposition of p-EA. Molecular-resolution STM imaging revealed that poly(m-EA) and poly(o-EA), denoted respectively as m- and o-PEA, exhibited distinctively different molecular shapes. More specifically, m-PEA molecules were predominantly linear and aligned preferentially in the 〈121〉 directions of the Au(111) surface; whereas o-PEA molecules were ill-defined in shape and in dimension. These differences in molecular conformation stemmed from unlike arrangements of adsorbed monomers at 0.9 V. Notably, m-EA were adsorbed in zigzags with two nearest neighbors separated by ∼0.5 nm, which were spatially so similar to the backbones of m-PEA that m-EA molecules coupled readily when the potential was raised high enough to induce the oxidation of m-EA. In contrast, the arrangement of o-EA molecules was so different from the ideal configuration of its polymer that molecules coupled randomly to yield crooked polymer chains less than 20 nm in length. The effect of potential on the structure of m-PEA was examined also, revealing notable branching of linear m-PEA if the electrochemical potential was set at 1.1 V.