We present a detailed study of the thermal stability of organic thin films of diindenoperylene encapsulated by sputtered aluminum oxide layers. We studied the influence of capping layer thickness, stoichiometry, and heating rate on the thermal stability of capped films and their eventual breakdown. Under optimized encapsulation conditions (thick and stoichiometric capping layer), the organic films desorb only at temperatures 200 °C above the desorption of the uncapped film. Moreover, the capped organic films retain their crystalline order at these elevated temperatures, whereas they would normally (i.e., uncapped) be in the gas phase. This study therefore also shows a way of studying organic materials under temperature conditions normally inaccessible. Considering results from complementary techniques, we discuss possible scenarios for the eventual breakdown. The results have implications for the performance and long-term stability of organic devices for which stability against elevated temperatures as well as against exposure to ambient gases is crucial.