Structures of five new O-specific polysaccharides of Proteus bacteria were established. Four of them, Proteus penneri 4 (O72), Proteus vulgaris 63/57 (O37), Proteus mirabilis TG 277 (O69), and Proteus penneri 20 (O17), contain O-acetyl groups in non-stoichiometric quantities, and the polysaccharide of P. penneri 1 is structurally related to that of P. penneri 4. The structures were elucidated using NMR spectroscopy, including one-dimensional 1H- and 13C-NMR spectroscopy, two-dimensional 1H,1H correlation (COSY, TOCSY), H-detected 1H,13C heteronuclear multiple-quantum coherence (HMQC), heteronuclear multiple-bond correlation (HMBC), and nuclear Overhauser effect spectroscopy (NOESY or ROESY), along with chemical methods. The structural data obtained are useful as the chemical basis for the creation of the classification scheme for Proteus strains.