Abstract—The structures of exo-oligosaccharides produced by a symbiotic nitrogen-fixing bacterium Rhizobium leguminosarum bv. viciae VF39 were determined using the methods of mass spectrometry and HCD fragmentation of ions. Exo-oligosaccharides were shown to be monomers, dimers and trimers of an octasaccharide repeating unit of the high-molecular-weight acidic exopolysaccharide. The major fraction was represented by octasaccharide monomers with two glucuronic acid residues at the reducing end. Heptasaccharides corresponding to an octasaccharide unit without GlcA-c or Glc-b residues in the main chain were also found in the culture liquid. Heterogeneity of exo-oligosaccharides according to the content of O-acetyl and hydroxybutanoyl substituents was revealed.