The influence of thermal and mechanical pretreatment on the phase transition behaviour of Polyamide-6 (PA 6) has been investigated by means of differential calorimetry and wide angle X-ray scattering (WAXS). The crystallization of PA 6 during slow cooling or quenching is facilitated when shear forces have acted on the melt before, DSC heating curves of samples which have been oriented by precipitating from a formic acid solution or by shear forces acting on the melt prior to crystallization show three endotherms (T-m*Tm1Tm2). By means of dynamic WAXS investigations (HASYLAB, DESY) it could be shown for the first time that under certain conditions the Brill transition takes place in PA 6 crystallites and that it happens continuously during heating and cooling, DSC curves do not indicate the Brill transition, Synchroton radiation experiments show that under the chosen experimental conditions it is a ＇＇reversible＇＇ process.