Poly(ethylene terephthalate-co-isophthalate) (PETI) prepolymer was submitted to solid state polymerization (SSP) at 184230 degrees C in a fixed bed reactor, to study the evolution of morphological changes during the process. Short reaction times were selected to investigate crystallization phenomena during nonisothermal (heating) and isothermal SSP phases. More specifically, multiple PETI melting behavior was observed and attributed to secondary crystallization, the rate of which increased significantly with SSP temperature. Reaction time was also found to exert a positive effect on solid-phase perfection of secondary crystals, leading at each temperature to melting points close to the value of bottle-grade poly(ethylene terephthalate). Finally, the mass fraction crystallinity of the SSP grades was found to comply with the crystal morphology encountered. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011