Copolyesters made from Poly(butylene terephthalate) (PBT) and p-acetoxy-benzoic acid (PAB) have been examined in solution by proton nuclear magnetic resonance (NMR) spectroscopy. Proton NMR spectra of solutions have shown that the sequence distribution for POB/PBT copolyesters can be described in terms of a probability model in which POB (p-oxybenzoate) has a chance that is almost random of being bonded to another POB in the copolyesters, with POB content ranging from 20 to 40 mol %. From the experimental data, we can predict that the higher the content of POB in copolyester is, the larger the deviation from randomness of sequence distribution is.