Polysaccharide chains of P. aeruginosa O5a, b, c, O5a, b, d and O5a, d (Lányi classification) lipopolysaccharides contain D-xylose, N-acetyl-D-fucosamine (FucNAc) and a derivative of 5,7-diamino-3,5,7,9-tetradeoxy-L-glycero-L-manno-nonulosonic acid (pseudaminic acid, PseN2) carrying acetyl or (R)-3-hydroxybutyryl (Hb) and formyl (Fm) groups as N-acyl substituents. Degradation of the lipopolysaccharides with dilute acetic acid caused depolymerisation of the polysaccharide chains as a result of cleavage of glycosidic linkage of pseudaminic acid to give trisaccharides representing chemical repeating units of the polysaccharides. Basing on analysis of the trisaccharides using 1H and 13C NMR spectroscopy and mass-spectrometry, the following structures of the polysaccharide chains were established: (Formula: see text). O5a, d polysaccharide is identical to P. aeruginosa immunotype 6 O-specific polysaccharide.