The microstructure of poly(butylene terephthalate) (PBT) nanocomposites was investigated by simultaneous small angle X-ray scattering/wide angle X-ray scattering (SAXS/WAXS) measurements at room temperature. The PBT was observed to crystallize in the alpha-phase. The dispersion of single-wall carbon nanotubes (SWCNTs) in PBT, using in situ polymerization, materials with higher degree of crystallinity than neat PBT were produced. SAXS results indicated that the SWCNT may be preferentially distributed in the amorphous phase of PBT, although WAXS results suggested a nucleation ability of SWCNT, which was supported by the DSC results. Much more complex changes were induced by the dispersion of multiwall carbon nanotubes (MWCNTs) in the PBT matrix. Evidence for the formation of an interphase with restricted chain mobility were found by dynamical mechanical thermal analysis (DMTA). Differential scanning calorimetry (DSC) and WAXS showed an increase of the crystallinity of the nanocomposites in comparison to neat PBT. POLYM. ENG. SCI., 50:1571-1576, 2010. (C) 2010 Society of Plastics Engineers