The thickness of thin films of non-functional silane bis-1,2-(triethoxysilyl)ethane (H5C2O)(3)Si-CH2CH2-Si(OC2H5)(3) (BTSE) deposited on aluminium surfaces is investigated using spectroscopic ellipsometry (250-1700 nm). The data processing of the ellipsometry spectra is carried out by means of simulation and regression techniques. New advances in data processing, e.g. multiple sample analysis and determination of thickness non-uniformity, are applied to characterise these thin polymer films realistically. The influence of the concentration of the BTSE solution and the curing of the film is investigated. Optical thickness estimates are corroborated by independent auger electron spectroscopy and transmission electron microscopy analysis.