Template-free electrochemical deposition of large arrays of uniform and oriented conducting polymer nanowires has recently been demonstrated. In this article, we build upon the key earlier findings and report new phenomena (initial formation of a 2D film) and mechanistic insight into the growth of the nanowires. High-resolution scanning electron microscopy was used to investigate the initial nucleation and growth of polyaniline nanowires and fibrils on conducting electrodes. Nanowire formation was found to occur in three stages. First, nucleation sites are deposited onto the surface of the electrode. Second, horizontal and vertical growth occurs in combination to form a compact lateral 2D layer containing small vertical polymer nodules. Lastly, growth occurs only in the vertical direction, extending the polymer nodules to form nanowires. Larger diameter polyaniline fibrils also grew in conjunction with the nanowires. In contrast to the ordered growth of the nanowires, fibrils undergo successive and multiple branching, leading to the formation of large area spaghetti-like deposits. A series of growth studies show that nanowire and fibril formation are highly sensitive to the electrode material and polymerization current. Platinum and gold exhibited uniform and oriented nanowire growth while stainless steel and indium tin oxide had poor nanowire growth and a high incidence of fibril growth. High polymerization currents favored the formation of fibrils on all electrode materials. Crown Copyright © 2008.