We performed temperature-dependent studies on pentacene thin film transistors (TFTs) with and without encapsulation. The capping layer is realized either by a sputtering layer of aluminum oxide (AlOx.) or, alternatively, by a polymeric layer of poly-para-xylylene (PPX). A field-effect can be demonstrated for both capping materials up to temperatures of about 140 – 170 °C, which is about 50 °C above the desorption point of uncapped pentacene thin films on SiO2 substrates. Complementary studies by thermal desorption spectroscopy and temperature dependent x-ray diffraction show that the organic layer remains crystalline on the substrate far above the electrical breakdown temperature of the encapsulated device.