The intensity changes in the CH2 absorption bands at 1460 (relaxed) and 1485 cm-1 (strained) bands in the FTIR spectra of poly(butylene terephthalate) [24968-12-5] as functions of strain are used to quant. represent structural transformation progress and may be correlated with mech. stress-strain diagrams and wide-angle x-ray results. Unlike x-ray diffraction measurements, IR spectroscopic changes characteristic of the phase transition, are obsd. at 2% strain and extend beyond 15% strain up to sample failure.