DIRLD (dynamic IR linear dichroic) spectra of prestretched isotactic polypropylene are recorded. The line shape features in the in-phase spectra are described on the basis of frequency shifts and absorption amplitude variations during the stretched cycle. Details such as the dichroism of the bands, direction of the shift, and changes in the absorption intensities are considered to explain signs and shapes of the DIRLD bands. Evidence for chain reorientations under stress has not been found. The general principle of frequency shifts and changes in absorption intensities as the origin of DIRLD bands is demonstrated by spectral simulations.