ZrO2 films with a thickness as low as 4 nm and a roughness of about 0.2 nm have been deposited on Si(001) by a sol-gel process. After pyrolysis in air clean and dense ZrO2 films were obtained. To simulate the influence of thermal processes in complementary metal-oxide-semiconductor fabrication on high-k gate oxides, our samples have been subjected to heat treatments up to 1000 °C. The chemical composition of the ZrO2 films and of the interface region has been monitored by Auger electron spectroscopy (AES) and AES depth profiles. No notable chemical changes in the interface region have been detected after heating at 700 °C in 2×10-5 mbar oxygen partial pressure and rapid annealing to 1000 °C. At 700 °C and 10-4 mbar oxygen partial pressure an intermediate interface layer starts to grow by oxidation of the Si substrate. Annealing above 700 °C in UHV leads to the destruction of the sample. Loss of oxygen is accompanied with the formation of islands containing Zr and Si and of holes extending up to 200 nm deep into the Si substrate.