Semiconducting copper(I) oxide nanowires were fabricated by electrochemical deposition into porous polycarbonate membranes from a copper lactate electrolyte. Nanowire composition and structure were varied by controlling the deposition potential and deposition rate, respectively. Nanowires formed at more negative potentials had increased metallic Cu content leading to Cu2O/Cu composites. The nanowires were highly resistive but displayed enhanced conductivity under UV/visible light illumination. Optical spectra show both a plasmon resonance due to metallic copper and an interband absorption due to the bandgap of copper(I) oxide, calculated at 2.28 +/- 0.02 eV. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3208038] All rights reserved.