Thermally induced morphological changes in thin (1-5 mu m) films of poly(alpha,alpha,alpha＇,alpha＇-tetrafluoro-p-xylylene, (C6H4CF2)(n)) (Parylene-F or PPX-F) are characterized using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and thermal stress measurements. A reversible crystalline phase transition is observed between 360 and 400 degrees C, which to our knowledge has not been reported previously. The transition is accompanied by an increase of the in-plane tensile stress of the film, which is attributed to contraction of the in-plane polymer structure during the transition to the high-temperature crystalline form. The stress and thermal behavior are qualitatively similar to those occurring in the more extensively studied nonfluorinated material, poly(p-xylylene, (C6H4CH2)(n)) (Parylene-N or PPX-N), which undergoes a shift in stress during the beta(1)-beta(2) transition (270-300 degrees C).