Blends of poly(propylene) (PP) and poly(ethylene terephthalate-co-isophthalate) (co-PET) (95/5) with and without compatibilizing agent (maleic anhydride PP), as well as composites of these blends with glass beads (50 wt%) with and without silane coupling agent surface-treatment, were prepared and studied on a basis of the material microstructure and thermomechanical properties. Infrared and Raman spectroscopy, as well as transmission electron microscopy, displayed evidence of MAPP compatibilizing action for the blend. Differential scanning calorimetry showed a remarkable effect of nucleation rate increase exerted by co-PET on the PP crystallization. Moreover, glass beads were found to increase the PP nucleation rate slightly. PP crystallinity hardly varied with the composition. Wide angle X-ray diffraction allowed determination of differences in the orientation of the poly(propylene) b-axis, with more homogeneous orientations in the presence of both co-PET and glass beads. MAPP promoted the PP b-axis orientation. Differences in PP alpha' relaxation could be analyzed through dynamic-mechanical thermal analysis (DMTA). (C) 2004 Wiley Periodicals, Inc.