For many avionics applications, the possibility of operating fibers under high temperatures is desirable. Films of polymer fiber cladding were tested at 250°C for periods of up to 10 hours. During this heating, significant changes in Young's modulus, refractive index, weight, hardness, strength, elongation and Tg were observed, although some changes were much less than others. In our 150 °C study [2], in general, polymers with higher concentrations of fluorine, and thus lower refractive indices, were observed to perform better at higher temperatures for extended periods. We have seen results in some parameters at the much higher temperature of 250°C where the behavior is similar, but this is not always the rule. In some tests, such as refractive index change at 633 and 852 nm, materials with less fluorine were observed to demonstrate lower index change. As the degradation temperature of all the polymers examined in this paper falls between 250 and 280°C, that the changes in materials occur are not surprising. While these polymers cannot function at 250°C for long periods, it is possible to expose them for much shorter periods of time without apparent damage. Such polymers have been exposed to high temperatures for a few minutes while being successfully jacketed with extruded materials (including Tetzel®) [3]. Certain results at 4 and 6 hours showed small changes. A sensor or other fiber device might be able to function in these temperatures for an hour or more without the coating or device breaking down.