Polysaccharides composition of the tropical brown seaweeds Turbinaria turbinata, Sargassum filipendula, Dictyota caribaea and Padina perindusiata collected at Yucatan Peninsula (Mexico) was determined in this study. Crude fucoidan extracted with HCl and alginate extracted with a hot alkali solution were characterized in terms of their molecular weight, sulfate content, uronic acid, total carbohydrate and neutral sugar components. Low molecular weight sulfated-fucoidan was the major component in all species studied. Fucoidan from T. turbinata and from D. caribaea were characterized as a homofucan, with fucose as the neutral sugar. Fucoidan from S. filipendula was composed of a galactofucan, and fucoidan from P. perindusiata was characterized as a heterofucan consisting of fucose, glucose and galactose. The Fourier transform infrared (FT-IR) spectra of fucoidan extracted from species studied indicated that the majority of sulfate groups are located at C-4 and to a lesser extent at C-2 and/or C-3 of the fucopyranose residues. This could be advantageous since several therapeutic effects have been reported for fucoidans with similar characteristics. FT-IR spectra from D. caribaea and P. perindusiata revealed the presence of O-acetyl groups in crude fucoidan, which could be potentially utilized as an immune stimulant. Molecular weight of alginate varied between 595 and 1301 kDa with similar uronic acid content in all species. Alginate M : G ratio inferred from FT-IR spectra suggests a high content of G-block in all species. Potential applications of these polysaccharides are discussed.