Because of its slowly crystallizing nature, poly(ethylene terephthalate) (PET) can be supercooled into an amorphous glass by rapid quenching. Upon reheating between T-g and T-m, the amorphous PET are subjected to two competing processes: rubber softening and crystallization. Fusion bonding of two such crystallizable amorphous polymer sheets in this processing temperature window is thus a complex process, different from fusion of purely amorphous polymer above T-g or semicrystalline polymer above T-m. In this study, the interfacial morphological development during fusion bonding of supercooled PET in the temperature window between T-g and T-m was studied. A unique double-zone interfacial morphology was observed at the bond. Transcrystals were found to nucleate at the interface and grow inward toward the bulk and appeared to induce nucleation in the bulk to form a second interfacial region. The size and morphology of the two zones were found to be significantly affected by the fusion bonding conditions, particularly the fusion temperature. The fusion bonding strength determined by the peeling test was found to be significantly affected by the state of crystallization and the morphological development at the bonding interface. Based on the interfacial morphology observed and the bonding strength measured, a fusion bonding mechanism of crystallizable amorphous polymer was proposed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3101-3112, 2011