The combination of Fourier transform Raman spectroscopy and thermal analysis has been proved to be adequate for the study of the quantitative structural changes which take place in amorphous poly(ethylene terephthalate) on stretching. The two-phase conformational model previously proposed for annealed samples was applied to uniaxially stretched samples. The samples were deformed at different temperatures and speeds. Different conformer contents were found in the stretched sample depending on the stretching conditions. In general, deformation induces a conformational transition of gauche to trans. This behavior was lost at a narrow range of temperatures above the glass transition and at the lowest speeds, where the stretching seemed not to induce conformational changes. This different behavior has been associated to the high mobility of the polymer chains under these conditions, which imposes a different process of deformation marked by a high degree of chain slipping. This kind of deformation is present all along the sample and with the absence of necking in the uniaxial drawing process.