Ulvan, the sulfated cell-wall polysaccharides from green seaweeds ( Ulva species), presents structural and functional properties of interest for different applications. Its extraction yield in water varies depending on the species, the period of collect and the mode of conservation of algae. To identify limits of extraction, the structure and interactions of ulvan in the cell wall of Ulva were investigated following a sequential solvent extraction. Hot sodium oxalate and hot dilute HCl were the two major extracts. Other solvents affecting protein structure or low hydrogen bonded polymers yielded minor ulvan fractions but affected solid state 13C NMR spectra indicating that proteins likely contributed to cross-link and/or to entrap Ulva cell wall polysaccharides. Ulvan-lyase degradation showed that ulvan consisted in a wide continuum of related polysaccharides differing in the extent and type of fine structural characteristics. Besides solvent acidity, particle size of seaweeds was an important parameter controlling the ulvan extraction efficiency. Major ulvan cell wall interactions were discussed with regard to ionic interactions, covalent linkages and peculiar solution behavior of ulvan in various physicochemical environments.