The molecular and crystal deformation of single fibers of PEEK (poly(aryl ether ether ketone)) has been followed using a combination of Raman spectroscopy and X-ray diffraction. The Molecular orientation of the fibers is characterized using polarized Raman spectroscopy, and it is shown that the high degree of molecular orientation is in agreement with crystal orientation revealed by X-ray diffraction. By using a submicron (500 nm) X-ray beam size to map across a single fiber diameter, it is shown that there is no difference between the orientation of the skin of the fiber and the core. Upon deforming the fibers in tension. Raman spectroscopy reveals that a number of hands, particularly the 1147 cm(-1) (C-O-C stretch) and the 1610 cm(-1) (phenylene ring stretch) modes, shift toward a lower wavenumber position, which is known to be indicative of direct molecular deformation. It is shown that these shifts in the Raman hand peak, positions are nonlinear with strain and stress and that they follow the form of the stress-strain curves obtained for the fibers. Two changes in slope are noted in the shifts of these peaks, which coincide with the yield points in the stress-strain curve. The magnitude of the initial slope of the shift in the 1147 and 1610 cm(-1) bands with tensile deformation is shown to be close to that reported for both cellulose and other p-phenylene rind, based fibers. Using X-ray diffraction, it is also shown that the crystals Undergo deformation during fiber stretching. The crystal modulus of the fibers is reported and shown to be in agreement With previously published data. In addition. it is shown that the uniform skin-core morphology is maintained during deformation.