Au nanoparticles are known to be efficient CO oxidation catalysts under ambient conditions. Investigation of the interaction of CO with Au is difficult particularly using Raman spectroscopy. Although indirect, we demonstrate the transfer of electrons from Au to CO by means of surface-enhanced Raman scattering of 2,6-dimethylphenyl isocyanide, exploiting the susceptibility of the NC stretching band of isocyanide to the variation of the surface potential of Au nanoparticles. The back-bonding from Au to the CO 2 pi(b) orbital must be dominant over the 5 sigma donation of CO to Au. (C) 2013 Elsevier B.V. All rights reserved.