IR spectra were continuously recorded during stretching of isotactic polypropylene [25085-53-4] film using a rapid scanning Fourier transform spectrometer. The results support the view that, in the necking region, the polymer chains of both amorphous and cryst. domains are oriented by a rigid-body mechanism involving translation and rotation of entire chains with respect to the direction of stretch. However, in the strain-hardening region at >250% strain further orientation is reflected only by absorption bands involving vibrations of the polymer backbone or which belong to conformationally regular sequences. The polarization properties of the skeletal vibrations can be accounted for by the extensional orientation mechanism proposed by R. P. Wool (1976). The polarization behavior of the 999 cm-1 band reveals that in the strain-hardening region further chain alignment occurs preferentially for conformationally regular segments.