Electrospun poly(ethylene terephthalate) (PET) fibers were prepared from a trifluoroacetic acid (TFA)-based solvent. Rheological studies revealed the concentration (phi) dependence of the specific viscosity (eta(sp)) to be eta(sp) similar to phi(3.7) for PET/TFA solutions in the entangled regime. The determined entanglement concentration (phi(e)) was higher using a lower-molecular-weight PET. To obtain bead-free fibers, the minimum concentration for the electrospinning was 0.8-1.0 phi(e), owing to the high volatility of TFA solvent, which significantly enhanced the chain network strength during jet whipping. The double-logarithmic plots of the jet (d(j)) and fiber (d(f)) diameters versus the zero-shear viscosity (eta(o)) revealed that two scaling laws existed for the present solutions, i.e., d(j) similar to eta(0.06)(o) and d(f) similar to eta(0.37)(o). The microstructural evolution of the electrospun PET fibers from stepwise annealing to crystal melting was investigated by simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation sources. The conformer transformation from gauche to trans was monitored by in-situ Fourier transform infrared spectral measurement. In the absence of any WAXD reflection, the as-spun PET fibers possessed a SAXS scattering peak, indicating the presence of a mesomorphic phase with an interdomain distance of 6.8 nm. At annealing temperatures (T-a) higher than 100 degrees C, the mesomorphic phase gradually transformed into imperfect triclinic crystals and reached its saturation at 130 degrees C. Further increased T-a perfected the triclinic structure without altering fiber crystallinity until the initial crystal melting at 218 degrees C, at which a significantly increased long period was detected. When the electrospun PET fibers were embedded in an isotactic polypropylene (iPP) matrix, surface-induced crystallization occurred to develop a transcrsytalline layer of iPP monoclinic crystals at the interface.