Using a 2,2,2-trifluoroethanol/deuteriochloroform solvent mixture (TFE/CDCl3), well-resolved solution C-13 NMR spectra of nylon 66 samples were obtained that showed several low-intensity peaks not seen in other, more acidic solvents. These low-intensity peaks were correlated to the methylene carbons near cis amide conformers, acid and amine end groups, and within cyclic oligomers. On the basis of relative peak intensities, the quantitative contents of these components for a typical sample of nylon 66 were determined to be 1.1 mol % cis amide conformers, 1.0 mol % acid end groups, and 0.5 mol % amine end groups. A number-average molecular weight of 30 177 g/mol was calculated from end group concentrations, which is a typical value for commercial nylon 66 samples. The chemical shift values of methylene carbons contained in end groups and in cyclic oligomers were found to be pH sensitive and therefore dependent on the relative concentrations of acid and amine end groups present in each sample. Understanding this behavior, in conjunction with the peak assignments described here, allows identification and quantitation of structural units and contaminants important in controlling chemical and physical properties.