The deformation behavior of poly(ether ester) is studied by means of small- and wide-angle X-ray scattering (SAXS and WAXS). The material under investigation represents a polyblock thermoplastic elastomer of poly(ether ester) (PEE) type. It comprises poly(butylene terephthalate) (PET) as hard segments and polyethylene glycol (PEG) as soft segments in a ratio of 57/43 wt Sb. Isotropic PEE bristles are drawn to five times of their initial length and subsequently annealed with fixed ends for 6 h at 170 degrees C in vacuum. The WAXS patterns were registered by a pinhole camera and a 2D area gas detector. These measurements were performed both under stress and during the subsequent relaxation in the absence of stress. The deformation was increased stepwise up to the breaking point of the sample (ca. 185%). SAXS patterns were obtained in the same deformation range by means of monochromatic X-ray radiation in the beamline A2 of the synchrotron DESY in Hamburg, Germany. SAXS patterns were registered by means of a 2D "Image-plate" detector. Five deformation intervals were revealed by SAXS. In the first one (epsilon = 0-50%) an ensemble of uncorrelated strained microfibrils exists and the corresponding layer Line small-angle pattern is observed. These microfibrils scatter independently. In the second interval (epsilon = 50-80%) interactions between neighboring microfibrils develop, a microfibrillar network is observed, and the layer line pattern transforms into a four-point diagram. In the third interval (epsilon = 80-100%) two additional reflections show up and an unique six-point pattern is seen. Pull-out of tie molecules from crystallites begins to fibrillate the network. This pull-out mechanism is independently proved by WAXS. In the fourth interval (epsilon = 100-130%) the mean long period of the four-point pattern decreases and the pattern itself vanishes. At last the fiber is completely fibrillated and only a two-point pattern remains visible. In the second, third, and fourth intervals, the microfibrils correlate in the transverse direction, which allows determination of the interfibrillar distance (so-called transverse long period). It decreases gradually with the progress of deformation in both the strained and relaxed state. In the fifth interval (epsilon > 130%) the long period of the two point pattern remains constant, but its intensity decreases until the fiber breaks. Only a few microfibrils are simultaneously carrying the load. They are destroyed one by one, until the fiber breaks as a whole.