The formation of interface Cu(In,Ga)Se-2/CdS in solar cells is not yet well understood but seems to be the key to further improvements of their performance. This interface depends on many parameters such as the initial chemical state of the CIGS surface or the chemical bath deposition conditions used to grow the US layer. In this paper, we focus our attention on the CIGS/US hetero-interface at different stages of its formation using mainly XPS studies of buried interfaces which were studied after gradual sputtering. We have investigated interfaces on CIGS submitted to various surface treatments, analogue to those involved in fabrication steps (NH3 dipping...). Kelvin probe and admittance spectroscopy measurements have been also performed on several interfaces prepared in the same conditions to correlate the chemical composition with the electrical response of the buried interfaces. (C) 2003 Elsevier Science B.V. All rights reserved.