The influence of the development of crystalline structure on the segmental dynamics of the amorphous phase in poly(butylene isophthalate) (PBI) has been studied by a combination of relaxation and scattering techniques. By means of dielectric spectroscopy (DS) the dynamics of both amorphous and semicrystalline PBI samples has been observed in a wide frequency and temperature range. The evolution of the crystalline phase with time has been studied in a range of temperatures, starting from initially glassy PBI by simultaneous small and wide angle X-ray scattering (SWAXS). By using a state-of-the-art setup designed specifically for the in situ study of both DS and SWAXS simultaneously (SWD), the crystallization of initially amorphous PBI has been followed in real time from both the structural and dynamics points of view. The obtained results support a model based on two different regimes on crystallization and a heterogeneous distribution of lamellar stacks. During the first regime the primary stacks cause the apparition of a rigid amorphous phase (RAP), i.e. a phase of amorphous chains lacking segmental motion. During the second regime, however, no more RAP is observed, indicating that the new lamellae that appear during this stage are nearly individual and not forming lamella stacks. (c) 2005 Elsevier Ltd. All rights reserved.