The structure of the carbohydrate backbone of the lipooligosaccharide (LOS) of the marine bacterium Arenibacter certesii strain KMM 3941(T) has been elucidated. The structure was obtained by means of compositional analysis, matrix-assisted laser desorption/ionization mass spectrometry and complete 1H and 13C and 31P NMR spectroscopy. It shows novel and interesting aspects and is the first description of Arenibacter lipopolysaccharides. Strong and mild alkaline treatments, to fully deacylate and only to O-deacylate the LOS were performed in order to determine the core structure. The core consists of a mixture of species differing by the presence of a non-stoichiometric alpha-d-rhamnose residue. The Kdo unit is substituted at O-5 by alpha-mannose and at O-4 by a alpha-galactosyluronic acid phosphate. The lipid A is constituted by a bis-phosphorylated disaccharide unit composed by a 2,3-diamino-2,3-dideoxy-beta-d-glucopyranose (DAG) residue as non-reducing end and a GlcN as reducing end.