Kinetics of electron transfer of the Cu(I)/Cu(II) redox couple at a platinum electrode has been studied with chronoamperometry, cyclic voltammetry and impedance spectroscopy in a deep eutectic solvent consisting of choline chloride and ethylene glycol. At 25 C, the reaction was found to be quasi-reversible with a relatively high rate constant k(0) of 9.5 +/- 2 x 10(-4) cm s(-1), and a charge transfer coefficient alpha of 0.25 +/- 0.05. Diffusion coefficients for the Cu(I) and Cu(II) complexes were determined to be 2.7 +/- 0.1 x 10(-7) and 1.5 +/- 0.1 x 10(-7) cm(2) s(-1), respectively. The viscosity of the electrolyte was 41 +/- 3 mPa s. The temperature dependency was also investigated. The activation energy of mass transfer was found to be 27.7 +/- 1 kJ mol(-1) and that of electron transfer 39 +/- 7 kJ mol(-1). Speciation of the Cu(I) and Cu(II) complexes was determined using UV-VIS spectroscopy, and the prevailing Cu(I) complex was found to be [CuCl3](2-) and that of Cu(II) [CuCl4](2-). (C) 2011 Elsevier Ltd. All rights reserved.