Commercial polyethersulfone (Udel (R) 1800) based on 4,4'-dihydroxy-2,2-diphenylpropane (bisphenol-A) and dichlorodiphenylsulfone was post-sulfonated using trimethylsilyl chlorosulfonate under mild conditions. The partially sulfonated polyethersulfones were investigated by H-1 and C-13 NMR in their acid and/or sodium salt form. Several C-13 NMR signals could be assigned up to the triad level. C-13-T-1 relaxation times were determined by the inversion recovery method for all carbons to ensure conditions for quantitative C-13 NMR measurements. Additionally, the nuclear Overhauser enhancements are given. The degree of sulfonation (DS) of the samples covers the range from 13.6 to 100% as determined both by H-1 and C-13 NMR spectroscopy. The NMR spectra confirm that the sulfonation occurs solely as monosubstitution of each phenyl ring of the dioxy-2,2-diphenylpropane unit in ortho-position to the ether bond. The microstructure of all polymers was evaluated from the content of dioxy-2,2-diphenylpropane-based diads and diphenylsulfone-centred triads. From the development of non-, mono- and disulfonated dioxy-2,2-diphenylpropane units with increasing DS it can be concluded that the sulfonation of the first phenyl ring retards the sulfonation of the second one. Thus, the non-sulfonated units deplete faster and the monosulfonated units are enriched compared with a random sulfonation. There is no reactivity influence between dioxy-2,2-diphenylpropane units which are separated by a diphenylsulfone unit because the content of diphenylsulfone-centred units follows the random distribution. (C) 2012 Elsevier Ltd. All rights reserved.