The exocellular polysaccharide produced by the cyanobacterium Cyanospira capsulata has been subjected to partial acid hydrolysis and N-deacetylation-nitrous acid deamination. The oligosaccharides released have been isolated by weak anion exchange and aqueous size exclusion chromatography, and characterized by a combination of 1D and 2D nuclear magnetic resonance spectroscopy, mass spectrometry, sugar composition and linkage analyses. The polysaccharide has an octasaccharide repeating unit with the following structure: [formula: see text]