The electrochemical behavior of CdS and Cu(I)-doped CdS nanoparticles were investigated in the solid state. The appearance of new peaks and the decrease of the currents with the potential scan the cyclic voltammetry of CdS and CdS:Cu(I) nanoparticles indicate that the redox of the nanoparticles is coupled with complicated chemical reactions. The coupled chemical reactions lead to the loss of the electrochemical activity for both kinds of nanoparticles. Faraday currents from the square wave voltammetry show that the redox of CdS is quasi-reversible, while the redox of CdS:Cu(I) is irreversible. The electrochemical decomposition of CdS and CdS:Cu(I) brings the difficulty to obtain clear electrochemical profiles without the interference from the coupled chemical reactions. Nonetheless, the HOMO levels (the valence band) of CdS and CdS:Cu(I) nanoparticles can be derived from the cyclic voltammetry. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.064302jes] All rights reserved.