Quantitative image analysis of transmission electron micrographs (TEM) of poly(ethylene terephthalate) (PET) thin sections was performed and compared to data obtained by small-angle X-ray scattering (SAXS) on the same samples. Different methods to analyze the TEM images are presented, and the results are compared with different ways to analyze the SAXS patterns. The results from TEM and SAXS correlate well and lift remaining ambiguities on the semicrystalline morphology formed on cooling from the melt, as a function of the isothermal crystallization temperature. A broad monomodal distribution of lamellar thickness was observed, while no large amorphous gaps were detected. The mean lamellar thickness was shown to be smaller than the mean thickness of amorphous regions, which was also characterized by a broad distribution.