The dynamic mechanical and dielectric relaxation properties of a commercial poly(trimethylene terephthalate) [PTT] have been investigated for both quenched and isothermally melt-crystallized specimen films. The relaxation characteristics of PTT were consistent with those of other low-crystallinity semiflexible polymers, e.g. PET and PEEK. While the sub-glass relaxation was largely unperturbed by the presence of the crystalline phase, both calorimetric and broadband dielectric measurements across the glass transition indicated the existence of a sizeable rigid amorphous phase (RAP) fraction in melt-crystallized PTT owing to the constraining influence of the crystal surfaces over the crystal-amorphous interphase region. A strong increase in measured dielectric relaxation intensity (Delta epsilon) with temperature above T-g indicated the progressive mobilization of the RAP material, as well as an overall loss of correlation amongst the responding dipoles. (c) 2006 Elsevier Ltd. All rights reserved.