The acidic O-specific polysaccharide of Proteus vulgaris O22 was studied using 1H and 13C NMR spectroscopy, including 2D COSY, TOCSY, NOESY, and H-detected 1H, 13C heteronuclear multiple-quantum coherence (HMQC) experiments, and the following structure for the branched pentasaccharide repeating unit was established: [sequence: see text] where Qui3NAc is 3-acetamido-3,6-dideoxyglucose, O-acetylation of QuiNAc at position 4 is stoichiometric and at position 2 nonstoichiometric. Serological relationships of P. vulgaris O22 with some other Proteus strains were substantiated on the level of the O-antigen structures.