Sulfated fucans from marine invertebrates have simple, linear structures, composed of repeating units of oligosaccharides. Most of these polysaccharides contain 3-linked fucosyl units, but each species of invertebrate has a specific pattern of sulfation. No specific enzyme able to cleave or to desulfate these polysaccharides has been described yet. Therefore, we employed an alternative approach, based on mild acid hydrolysis, in an attempt to obtain low molecular-weight derivatives from sulfated fucans. Surprisingly, we observed that sulfated fucans from Lytechinus variegatus and Strongylocentrotus pallidus (but not the sulfated fucans from other species) yield by mild acid hydrolysis oligosaccharides with well-defined molecular size as shown by narrow bands in polyacrylamide gel electrophoresis (PAGE). The sulfated oligosaccharides obtained by mild acid hydrolysis were purified by gel-filtration chromatography, and their structures were identified by (1)H-nuclear magnetic resonance (NMR) spectroscopy, revealing an identical chemical composition for all oligosaccharides. When we followed the acid hydrolysis by (1)H-NMR spectroscopy, we found that a selective 2-desulfation occurs in the fucans from S. pallidus and from L. variegatus. The reaction has two stages. Initially, 2-sulfate esters at specific sites are removed. Then the desulfated units are cleaved, yielding oligosaccharides with well-defined molecular size. The apparent requirement for the selective 2-desulfation is the occurrence of an exclusively 2-sulfated fucosyl unit linked to or preceded by a 4-sulfated residue. Thus, a homofucan from Strongylocentrotus franciscanus resists desulfation by mild acid hydrolysis, because it lacks the neighboring 4-sulfated unit. Overall, our results show a new approach for desulfating sulfated fucans at specific sites and obtaining tailored sulfated oligosaccharides.