High field H-1 and C-13 NMR spectroscopy and published information in the scientific literature were combined in the determination of the chemical structure of the commercially available Torlon(R) 4000T, a polyamide-imide which is widely used in applications where good mechanical, thermal and oxidative properties are required. It was found from qualitative and quantitative NMR analysis that the three monomers used to synthesize Torlon are as follows: 1.0 trimellitic anhydride chloride, 0.7 4,4'-oxydianiline, 0.3 m-phenylenediamine. It was also shown that the sequence of appearance of the polymer's amide and imide functional groups follows a certain distribution of Amide-Imide, Amide-Amide and Imide-Imide repeat units due to the dual reactive sites, acid chloride and anhydride, of the monomer trimellitic anhydride chloride. Crown Copyright (C) 2003 Published by Elsevier Ltd. All rights reserved.