Self-assembled monolayers (SAMs) of omega-substituted alkanethiols on gold display different effects on the kinetics of electrochemical deposition of polypyrrole and polyaniline. Hydrophobic monolayers of dodecanethiol and octadecanethiol initially retard the deposition of polypyrrole. However, as soon as polymerization is nucleated, a rough polymer film is rapidly deposited. Monolayers of 12-amino-1-dodecanethiol, which are protonated under the acidic conditions required for polyaniline deposition, provide a good barrier to aniline oxidation. The response of SAM-modified electrodes to (N,N-(dimethylamino)methyl)ferrocene in acidic solution follows similar trends observed for the blocking of aniline oxidation. Thus, the Coulombic repulsion between the protonated monolayer and the diffusing anilinium monomer appears important in providing a barrier to the electrooxidative deposition of polyaniline.