In this paper, a study of the structural changes due to annealing of injection-moulded polyamide-6,6 has been carried out. The spectroscopic behaviour of polyamide-6,6 is quite similar to that of poly(ethylene terephthalate) (PET). Using the band at 1650 cm-1 as an internal reference band, the intensity changes of the bands situated at 1146 and 936 cm-1 were followed. The former decreases when the annealing temperature exceeds 180-degrees-C whereas the latter increases. Furthermore, the bandwidth of the band at 936 cm-1 decreases from 23 to 20 cm-1 for annealing above 180-degrees-C. These spectroscopic changes were related to gauche/trans isomerism induced by the thermal treatment. Moreover, polyamide-6,6 verifies a two-phase conformational model, similarly to PET. As far as the thermal behaviour is concerned, two endothermic peaks at low (LM peak) and high (HM peak) temperature were found in thermograms of samples annealed above 150-degrees-C, and these displayed similar behaviour to those found in PET. The LM peak can be attributed to melting of crystals with increasing perfection and fold-surface smoothing of the crystalline layers due to the annealing treatment, and the HM peak to melting of the recrystallized crystalline structure during heating in differential scanning calorimetry (d.s.c.). On the other hand, the influence of the fabrication process in polyamide-6,6 seems not to be as important as in PET. The correlation between Fourier-transform infra-red spectroscopy with photoacoustic detection (p.a.-FTi.r.) and d.s.c. measurements show that the amide group works as an important constraint that limits the mobility of the crystalline molecules, and most of the conformational changes occur in the amorphous phase for annealing temperatures above 180-degrees-C. In the ordered phase, a slight crystallinity increase beyond this temperature can be related to crystalline perfection and fold-surface smoothing.