The polysaccharide from Cladhymenia oblongifolia and some of its derivatives have been examined by 13 C nuclear magnetic resonance spectroscopy and the spectra analysed with the techniques of set theory. The polysaccharide is an agaroid with significant levels of sulfation at most positions other than the axial hydroxyl group. The 4-linked units consist of anhydrogalactosyl and galactosyl units in the approximate ratio of 3:2. The galactose, but not the anhydrogalactose, units in the natural polymer have high levels of 2-sulfate ester, while low levels of galactose-3-sulfate are also present in the galactosyl residues. The 4-linked galactosyl units also contain significant levels of 6-sulfate, which is converted to anhydrogalactose by alkali treatment, including significant levels of anhydrogalactose-2-sulfate. The 3-linked units consist of high levels of galactose 2sulfate, moderate levels of galactose 6-sulfate, and very low levels of pyruvate acetal. Methylation is largely absent from this polysaccharide. The approximate structure can be summarized in set theory nomenclature as 3DGร2S 0.4 (4,6)P 0.05 6S 0.4 ร ฯช 4L{AG 0.6 , G 0.4 ร2S 0.35 3S 0.1 6S 0.3 ร}. The polysaccharide is similar to those extracted from other members of the Rhodomelaceae to the extent that it is a non-gelling agaroid, but the high levels of 2-sulfation are unusual as is the total variation in sulfation.