Medium infrared spectroscopy has been used by several authors to obtain the degree of crystallinity of PEEK films. The IR absorption bands at 947 and 966 cm-1, proposed by Jonas et al. are crystalline-sensitive. They appear as shoulders located in the neighborhood of other very intense absorption bands, and their deconvolution is difficult and sometimes hazardous for samples having a low crystallinity. In this same medium infrared region, designed hereafter as MIR, an additional band at 968 cm-1, appearing as a shoulder, is reported for the first time. The absorbance of these bands at 947 and 965 cm-1 does not approach zero for a density equal to the amorphous density. The analysis of the far infrared (FIR) spectrum of amorphous and partially crystalline PEEK films have indicated two ’crystalline-sensitive" bands at 218 and 303 cm-1, one "amorphous-sensitive" band at 410 cm-1, and an internal band at 272 cm-1. The integrated absorbance of the crystalline and amorphous-sensitive bands have been correlated to the macroscopic density and the crystallinity of the films. The presence of crystalline- and amorphous-sensitive vibrational bands in the FIR spectrum of PEEK allows us to use the method proposed by Hendus et al. These FIR measurements have been interpreted on the basis of a two-phase model, and no experimental evidence has been observed for a third additional phase.