The present study focuses on the durability of regular thick film materials for sensor applications in harsh water/steam environment by autoclave storage. Two experiments have been carried out. The first experimental run focuses on insulating materials typically used for thick film manufacturing. Two glass based overglazes, a glass based dielectric paste as well as the polymers PEEK and polyimide were tested in five consecutive autoclave experiments ranging from 110 °C/0.16 MPa to 287 °C/7.03 MPa, with a storage time of approx. 48 $h$ per run. Weight measurements and optical investigations after each experiment showed, that all glass-based insulators failed already during the 172 °C/0.83 MPa storage. The polyimide failed at 222 °C/2.37 MPa and the PEEK layer survived but started to lose adhesion to the substrate during the 172°C storage. It has been shown that the standard 96 % Al2O3thick film substrate is not suitable for this environment and shows strong degradation, while high purity 99.6 % Al2O3and ZrO2(Y2O3stabilized) resist even 290 °C/7.35 MPa. A second set of experiments focuses on the application of mostly metallic passivation layers for these vulnerable insulators. The results show that double layers of the investigated AgPt and Au thick film pastes are most reliable even at 290 °C/7.35 MPa, while single layers of AgPt and Pt paste offer no hermetic sealing and cause failure. Thin Ti films (300 nm and 500 nm) show good passivation capabilities at 152 °C/0.5 MPa but fail at 290 °C/7.35 MPa. The results indicate the difficulties of applying standard thick film technologies for sensor applications in a water/steam environment above 150°C and imply the necessity of new technology and material developments for such applications.