The performance of thin film microelectronic devices on SIMOX SOI substrates is highly dependent on the nature and perfection of the top-Si/BOX interfaces. In this research, we demonstrate a non-destructive and in-situ characterization method of spectroscopic ellisometry that is sensitive to these interfacial regions. The dielectric functions of the composite materials in the transition regions were calculated using the effective medium approximation (EMA). In addition, the interfacial roughness was also studied by TEM and AFM. (C) 2003 Elsevier B.V. All rights reserved.