The analysis of the triad sequence of a poly(ethylene/butylene terephthalate) copolymer using 600 MHz H-1 NMR is reported in a solvent system of o-chlorophenol/deuterated chloroform mixture (75/25 v/v) at 80 degreesC. The well-resolved alcoholic CH2 proton peak of the glycol units made the detailed sequence analysis possible. Exact peak assignment was performed with two special NMR experiments: a C-13-labeling experiment and a two-dimensional heteronuclear multiple bond correlation (HMBC) observation. The number-average sequence lengths calculated from. the triad sequences by 1H NMR were in good agreement with those from the dyad sequences by C-13 NMR. From the triad sequence information, it can be shown that the transesterification of the copolymer is controlled according to Bernoullian statistics.