The interface between an 'ion layer gas reaction' ZnO layer and a CuIn(S,Se)(2) thin film solar cell absorber has been investigated using UV- and X-ray photoelectron spectroscopy (UPS and XPS) and inverse photoemission (IPES). We compare interfaces with and without absorber-treatment in an aqueous ammonia hydroxide solution of Cd2+ ions prior to the ZnO deposition. By combining UPS, XPS and IPES, a direct and independent experimental determination of the valence and conduction band alignment at the heterojunction was performed. For the important conduction band offset we find that the absorber treatment leads to a transition from a cliff-like arrangement (-0.27 +/- 0.15 eV) to a small spike (+0.12 +/-0.15 eV). (C) 2003 Elsevier Science B.V. All rights reserved.