The conformational behavior of the algal polysaccharide ulvan in aqueous solution has been studied. This biopolymer, containing limited regular regions of aldobiuronic sequences made of beta-D-glcpA-(1-->4)-alpha-L-rhap 3-sulfate and of alpha-L-idopA(1-->4)-alpha-L-rhap 3-sulfate, can be considered as a low-cost source of rare sugars with a potential impact on several industrial applications, from pharmaceutical to fine chemistry industry. The relationship of structure with the chemical composition of the saccharidic residues in the biopolymer is evidenced with respect to the ability of iduronic acid moiety to assume two main ring conformations, namely the C-1(4) chair and the S-2(0) skew boat conformations. Experimental results are interpreted in the light of models obtained from conformational analysis calculations.