Electrochemical reduction of Cu(II) and electrodeposition of copper were studied in two room temperature protic ionic liquids (PILs), ethylammonium nitrate and triethylammonium methylsulfonate by using voltammetric techniques. The results provided evidence that slow disproportionation of Cu(I) occurred in the PILs. Images obtained by scanning electron microscopy revealed that electrodeposition of Cu nanoparticles and crystalline thin films onto glassy carbon and indium tin oxide electrodes occurred via two distinctly different, potential-dependent pathways: (i) disproportionation of Cu(I), at mild potentials in the range of 0.48 to 0.03 V vs. Fc(0/+), and (ii) electroreduction of Cu(I), at potentials lower than 0.03 V vs. Fc(0/+). The electrodeposition mechanisms were examined by using chronoamperometry and were found to be governed by instantaneous nucleation and diffusion-controlled growth kinetics. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.054210jes] All rights reserved.