Chemical structures of galactomannans from the native Brazilian species, M. scabrella, S. barbatiman and S. parahybum were studied. Their seeds, on aqueous extraction, furnished high yields of viscous galactomannans, whose mannose to galactose ratios were 1.1:1, 1.5:1, and 3.0:1, respectively. The polysaccharides were analysed by methylation, periodate oxidation and chromium trioxide oxidation. The results, confirmed by C-13 NMR spectroscopy, indicated expected structures of legume galactomannans, namely a polymeric main chain of (1-->4)-linked beta-D-mannopyranosyl residues substituted at 0-6 by single unit alpha-D-galactopyranosyl side-chains. C-13 NMR spectra showed clear splitting of the 0-substituted resonances of C-4 of the beta-D-mannosyl residues depending on the nearest-neighbour probabilities, indicating a random arrangement of the D-galactosyl groups in all of the three galactomannans.