The success of advanced IC interconnect schemes using copper depends on control of CU surface oxidation, since even a few monolayers of CuxO can dramatically increase Cu via resistance and reduce Cu-to-dielectric film adhesion. This level of control requires an accurate, in-line method of determining Cu oxidation state and CuxO film thickness. In this paper, we apply spectroscopic ellipsometry to characterize Cu films that were oxidized and then reduced under controlled thermal and plasma conditions representative of IC processing. Oxidation and reduction was carried out on single layers of sputter-deposited Cu as well as on Cu films deposited as part of multilayer thin film stacks. Ellipsometer models were developed and optimized for application to both Cu and copper oxide films. The correlation of ellipsometric data with Rutherford backscattering spectroscopy and x-ray fluorescence analysis of similar films is also discussed.