Si-diffusion from Si-based substrates into yttria-stabilized-zirconia (YSZ) thin films and its impact on their microstructure and chemistry is investigated. YSZ thin films used in electrochemical applications based on micro-electrochemical systems (MEMS) are deposited via spray pyrolysis onto silicon-based and silicon-free substrates, i.e. Si(x)N(y)-coated Si wafer, SiO(2) single crystals and Al(2)O(3), sapphire. The samples are annealed at 600 degrees C and 1000 degrees C for 20 h in air. Transmission electron microscopy (TEM) showed that the Si(x)N(y)-coated Si wafer is oxidized to SiO(z) at the interface to the YSZ thin film at temperatures as low as 600 degrees C. On all YSZ thin films, silica is detected by X-ray photoelectron spectroscopy (XPS). A particular large Si concentration of up to 11 at% is detected at the surface of the YSZ thin films when deposited on silicon-based substrates after annealing at 1000 degrees C. Their grain boundary mobility is reduced 2.5 times due to the incorporation of SiO(2). YSZ films on Si-based substrates annealed at 600 degrees C show a grain size gradient from the interface to the surface of 3 nm to 10 nm. For these films, the silicon content is about 1.5 at% at the thin film's surface. (C) 2011 Elsevier B.V. All rights reserved.