We report a study of the physical and electrochemical properties of two-component self-assembled monolayers (SAMs) composed of both electroactive (4-aminothiophenol, 4-ATP) and electroinactive (n-octadecanethiol, ODT) species. In all of the experiments reported here, relatively short (3 h) assembly times were used to prepare the mixed SAMs. We have characterized the macroscopic composition and the microscopic structure of these SAMs using Auger electron spectroscopy (AES), coulometry, grazing angle Fourier transform infrared spectroscopy, and lateral force microscopy (LFM). The adsorption isotherms determined by AES and coulometry show significant deviation from Langmuirian behavior and are suggestive of phase separation. LFM images obtained at three points near the critical region of the isotherm ([4-ATP]/[ODT] similar to 4) indicate that these two-component SAMs display complex phase behavior : At relatively low 4-ATP coverages, the surface consists of small islands of 4-ATP imbedded in an ordered film of ODT. At higher coverages of 4-ATP, however, we find evidence of both separation into distinct phases and mixing of the two components. In a second series of experiments, we demonstrate that phase domains of 4-ATP are electroactive and can be used to carry out localized electrochemistry. That is, the islands of 4-ATP, which are randomly distributed in the ODT matrix, behave as an array of ultramicroelectrodes. Surface-confined 4-ATP molecules can be used to nucleate the growth of polyaniline selectively from the phase separated domains of 4-ATP. We find that if a 4-ATP/ODT mixed monolayer is oxidized in the presence of aniline, nanometer scale polyaniline features are formed. The size and distribution of these features have been characterized by AFM and can be controlled through a combination of monolayer composition and the concentration of aniline in solution.