A NMR spectroscopic method was used for the determination of the diol contents such as propylene glycol and polyethylene glycol in copolyesters. The quantitative analysis of copolyester compositions was made by comparing the areas of the resonance peaks of the characteristic groups in the spectra. The compositions of the copolyesters made from dimethyl terephthalate with ethylene glycol and propylene glycol, containing whole range of propylene glycol, were determined from the ratios of phenylene vs. methyl protons. The compositions of the copolyesters made from dimethyl terephthalate with ethylene glycol and polyethylene glycol-400, containing less than 30 mole % of polyethylene glycol-400, could be determined from the differences in chemical shifts of ether- and ester-linked methylene protons of the diols, In order to confirm these results, the gas chrosmatographic method was used. The result stherefrom correspond to those proposed by NMR method.