The electrochemical behavior of the low-melting copper salts [Cu(MeCN)(x)][Tf2N] and [Cu(PhCN)(x)][Tf2N] (x = 2-4), where MeCN is acetonitrile and PhCN is benzonitrile, is presented. In these compounds, the copper(I) ion is a main component of the ionic liquid cation. Consequently, the copper concentration is the highest achievable for an ionic liquid and this permits to obtain a good mass transport and high current densities for electrodeposition. The cathodic limit of the ionic liquid is the reduction of copper(I) to copper metal instead of the breakdown of the cation as in conventional ionic liquids. It is shown that pure, crack-free copper layers can be deposited from these copper-containing ionic liquids in unstirred solutions at current densities up to 25 A dm(-2). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3512905] All rights reserved.