AbstractHydrocolloids from seaweeds have interesting functional properties, such as thickening or gelling ability. Structural characteristics of polysaccharides extracted from Québec seaweeds have not yet been established. Thus, the determination of the relationship between their structure and rheological behaviour is limited. Alginate and fucoidan were extracted using selective solvents from three species: Saccharina longicruris, Ascophyllum nodosum and Fucus vesiculosus. Structural analysis (total sugars, uronic acids, sulfates and molecular weight) and rheological characterisation were performed at different polysaccharide concentrations with and without the addition of NaCl. The results showed important variation between species. Fucoidan and alginate exhibited Newtonian behaviour. Fucoidan extracted from F. vesiculosus had the highest viscosity level, which might be explained by the degree of branching of the molecules. For alginate, the one extracted from S. longicruris showed a higher apparent viscosity. This result can partially be explained by the block proportion of alginate. The gelation profile of alginate was also determined for each species. The final storage modulus, G′, was variable for each species. Differences between species were observed for both polysaccharides as a result of structural variation. Copyright © 2007 Society of Chemical Industry