For the purpose of resolving an uncertainty over the correct determination of the crystalline lamellar thickness in semicrystalline poly(ethylene terephthalate), PET, via small-angle X-ray scattering (SAXS) analysis, a gel-crystallization method from oligomeric poly(ethylene glycol) solution was used to prepare samples with high crystallinity (57%). By using simultaneous synchrotron SAXS and wide-angle X-ray diffraction (WAXD) measurements, the heating and cooling processes of the gel-crystallized PET sample were monitored. Results support the assignment of the larger thickness value from the SAXS correlation function analysis as the lamellar crystal thickness. Analysis of WAXD 011 reflection line broadening gives the minimum lamellar thickness (in the chain axis) and verifies the thickness assignment for gel and melt crystallized samples. This assignment is critical as it affects the correct interpretation of the crystallization behavior in semicrystalline polymers of relatively low crystallinity.