Polyaniline thin films were electrodeposited onto naked Au and thiol-modified Au electrodes by potential cycling. Self-assembled monolayers (SAMs) contained 4-aminothiophenol (4-ATP) and n-decanethiol (DT) in varying molar ratios. The electropolymerization of aniline at these surfaces is strongly influenced by the composition of the monolayer. When the potential was cycled between 0 and 0.8V, the polymerization of aniline initiated at 4-ATP sites in the SAMs. The electrical conductivity of the films was found to increase as the mole fraction of 4-ATP in the SAM increased. When the potential was cycled between 0 and 1.0V, polymerization of aniline is initiated both at the surface and by the direct oxidation of aniline monomers in solution. Nevertheless, the composition of the SAM has a profound influence on the properties of the polyaniline films, especially the electrical conductivity, which increases with 4-ATP mole fraction up to a value of 0.77, and then decreases dramatically. The amount of 4-ATP in the SAM influences the rate of aniline oxidation, which determines the overall polymerization rate, and, in turn, influences the thin film conductivities. When the rate of monomer oxidation is too high, the films are over oxidized and lose conductivity. On the other hand, when the monomer oxidation rate is too low, the oligomer number suffers, and the conductivity drops. Thus, with respect to the electrical conductivity, there appears to be an optimum rate of aniline oxidation, which can be controlled by use of the appropriate SAM.