Strain-induced crystn. of natural rubber and the relation between segmental orientation and phase sepn. in polyester-urethane elastomers during elongation and recovery are studied by using Fourier transform IR spectroscopy. The onset of the crystn. of natural rubber is reflected in the stress-strain diagram by a significant increase of strain as a consequence of self-hardening. Spectroscopically, the crystn. corresponds to an increase in the intensity of the 1130 cm-1 absorption band which belongs to an in-plane deformational vibration of the Me group.