The origin of process-induced rectification in CdS/ITO and CdS/SnO(2) bilayers has been investigated. Both pre-treatment of the transparent conducting oxide (TCO) substrates and post-growth treatment of the bilayers were explored for both oxidising and reducing conditions. In/CdS/TCO structures were used for I-V testing, and the CdS layers were verified as being pinhole-free using a test employing a rectifying Au/CdS contact. Whilst neither pre-oxidation nor reduction of any TCO substrate failed to induce rectification in CdS/TCO, oxidation of CdS always induced rectification, regardless of the substrate type. This was attributed to oxidation of CdS (confirmed by Auger electron spectroscopy), and the results are consistent with a band diagram postulated for the CdO/CdS/ITO structure. Recommendations are made for the fabrication of CdTe/CdS/TCO solar cells. (C) 2010 Elsevier B.V. All rights reserved.