The contamination of heparin in 2008 brought to the attention of health authorities an urgent need for structural characterisation of low molecular weight heparins and other glycosaminoglycans (GAGs) intended for clinical applications. Potentially harmful compounds can be introduced into these preparations as contaminants of the original material or as by-products of the depolymerisation process. Radical depolymerisation is one of the methods used for fractionations of GAGs. We report here on the results of the Fenton-type radical depolymerisation of dermatan sulfate (DS) by hydrogen peroxide in the presence of Cu2+ cations. A low molecular fraction of the reaction mixture was investigated by a combination of 2D H-1, C-13 HSQC, 2D HSQC-TOCSY and 2D HMBC experiments at 800 MHz. The analysis of the spectra revealed the formation of oligosaccharides with structures corresponding to the native DS sequence and containing almost exclusively GalNAc4S as the reducing end monosaccharide. In addition. oligosaccharides containing a C-4 sulfated N-acetylgalactosaminic acid in place of the reducing end GalNAc4S were identified. This open chain monosaccharide represents a non-native DS structure. (C) 2011 Elsevier Ltd. All rights reserved.