AbstractUltrastructural analysis of the gel forming green seaweed sulfated polysaccharide ulvan revealed a spherical‐based morphology (10–18 nm diameter) more or less aggregated in aqueous solution. At pH 13 in TBAOH (tetrabutyl ammonium hydroxyde) or NaOH, ulvan formed an open gel‐like structure or a continuous film by fusion or coalescence of bead‐like structures, while in acidic pH conditions, ulvan appeared as dispersed beads. Low concentrations of sodium chloride, copper or boric acid induced the formation of aggregates. These results highlight the hydrophobic and aggregative behavior of ulvan that are discussed in regard to the peculiar gel formation and the low intrinsic viscosity of the polysaccharide in aqueous solution. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 652–664, 2009.This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com