Increasing demands on the optical performance of thin film coatings require a detailed knowledge of processes occuring during and after film preparation. Thin intermediate layers, needed in some layer stacks, are particularly challenging to describe. This makes exact predictions and understanding of the optical behaviour difficult, if based on optical spectroscopic data only. Therefore the knowledge gained by combining different characterization methods should elucidate new and mon effective pathways to tailor the optical properties of coatings. Optical spectroscopy (IR, Vis, UV) combined with simulations based on modelled dielectric functions, X-ray reflectometry (XRR), atomic force microscopy (AFM), Rutherford backscattering (RBS) and Auger electron spectroscopy (AES) have been used to characterize optical properties, layer thickness, roughness, and density and qualitative and quantitative element distribution of a single BiOx-film deposited on glass by DC-magnetron sputtering technique. A combination of these techniques has allowed a rather complete description of low-E systems and to understand why simulated optical spectra deviate from the measurements. In particular it is shown, that deviations between spectroscopic measurements and simulations are due to intermixing effects at one specific interface during the deposition process of a complete layer stack. The changes of optical properties by post-production heat treatment can be attributed to chemical reactions and segregation. This knowledge enables modifications which allow a considerable improvement in optical properties and performance.