Pentad-level sequence analysis of a copolyester derived from two diacids and two diols, poly(ethylene/trimethylene terephthalate/2,6-naphthalate), which was prepared by transesterification between poly(ethylene terephthalate) and poly(trimethylene 2,6-naphthalate), was reported in a solvent system of o-chlorophenol/deuterated chloroform mixture (75/25 v/v) at 80 degreesC using 600 MHz H-1 NMR. The well-resolved alcoholic CH2 proton peak of the glycol units was observed, which made the detailed sequence analysis possible. Many peaks were split into the pentad sequence centered on the glycol units and could be assigned by a comparison of the spectra of the model copolymers and by a two-dimensional heteronuclear multiple bond correlation observation. The pentad sequences determined here gave more detailed information on the change in the molar fractions of the sequences with increasing the transesterification reaction time. The discrimination among the different copolymerization state was performed on the basis of the pentad sequence analysis. Furthermore, the larger percentage of transesterification of the polymers was obtained than the value determined from the triad level analysis.