The crystal structure in random copolymers poly(ethylene terephthalate-co-4,4'-bibenzoate) (PET/BB) was studied using wide-angle X-ray diffraction (WAXD). The composition of crystalline and amorphous regions has been studied by FTIR spectroscopy using the amorphous peaks at 1580 and 1560 cm(-1) for terephthalate (T) and bibenzoate (B) units, respectively. Further, PET/BB55 was hydrolyzed to selectively etch the amorphous regions. The hydrolyzed products were further studied by FTIR and H-1 NMR spectroscopy, wide-angle X-ray diffraction, and transmission electron microscopy. Results show that crystal structures are very close to that of the respective homopolymer, PET and poly(ethylene bibenzoate) (PEBB) for the low and high BB content samples, respectively. The crystalline region of high BB content fibers is composed of mainly PEBB units. In PET/BB55 fiber, among the 30 wt % of the copolymer which was not etched, there is only one terephthalate unit for every five repeat units of bibenzoate. The crystal size of B type crystals along the c axis was found to be around five repeat units long. Short chain segments rich in BB are sufficient to induce crystallization, to form PEBB-like crystals.