The versatility and potential of rheo-optical FTIR spectroscopy is demonstrated with ref. to the characterization of segmental orientation in a polytetrahydrofuran-MDI-ethylenediamine copolymer elastomer. The rheo-optical data show that the soft and hard segments of the polymer deform by different orientation mechanisms. Thus, the soft segments exhibit an almost perfect reversibility of orientation and disorientation in loading and unloading cycles, whereas the deformation of the hard segments is dominated by a significant reorganization of their morphol. Rheo-optical FTIR spectroscopy provides an extremely sensitive tool for detecting the onset, progress, and decay of strain-induced crystn. of the soft segments during the mech. treatment of the elastomer.