The 1H-n.m.r. spectra of the Streptococcus pneumoniae type 9V (S68 in American nomenclature) capsular polysaccharide (PS) and its O-deacetylated derivative [which is structurally identical to the S. pneumoniae type 9A (S33) PS] were assigned using COSY, relayed-COSY, and 2D-NOESY experiments. The positions of the OAc groups in the alpha-GlcA, beta-ManNAc, and alpha-Glc residues of the native 9V PS were established using 2D-n.m.r. and chemical shift arguments, and the relative proportions of different O-acetylated species were estimated by integration of well-resolved 1H-n.m.r. signals. The locations of the OAc substituents differ from those previously reported. [formula: see text].