Marine green seaweeds synthesize acidic polysaccharides containing complex structures. Ulva species are producer of heteropolysaccharides named ulvans, which present the ulvanobiouronic acid 3-sulfate as major repeating disaccharide unit. On the other hand, Monostroma spp., biosynthesize sulfated rhamnans 2- or 3-linked, sulfated in C-2, C-3 or C-4. The best strategy to study these polysaccharides is to produce simpler structures representative of the native polymer. For this purpose we used the autohydrolysis approach for the partial degradation of these polysaccharides. In this study we determined the chemical structure of the polysaccharide produced by green seaweed Gayralia sp. The dry and milled alga was submitted to sequential aqueous extraction at 25ºC (twice) and 80ºC (four times) originating the following fractions Go1-Go6, respectively. Fraction Go4 was solubilized in water (1 g%) and submitted to cationic chromatography (Dowex 50 x 8, H⁺) to obtain the polysaccharide in the protonated form. Autohydrolysis was carried out at 80°C for 24 h and, after neutralization using NaHCO₃, the hydrolyzate was precipitate with ethanol (3v) and centrifuged. The supernatant and the precipitates gave rise to Go4HS (73%) and Go4HP (25%), respectively. The former fraction containing oligosaccharides was chromatographed on BioGel P2. The main subfraction (Go4HA) was submitted to anion exchange chromatography (DEAE-Sephacel) using water and (NH₄)₂CO₃ (0,05 - 1,00 M) as eluent. Fraction eluted with 0.05M (NH₄)₂CO₃ (Go4HAA2, 39.1% yield) contained rhamnose (90.4 mols%) and uronic acids (confirmed by ¹³C NMR signal at d 175.0). Additionally, ¹³C NMR spectrum showed signals at d 94.3 and d 94.9 attributed to b- and a- rhamnose reducing end, respectively. The major anomeric signals suggest the presence of 2- and/or 2-3-linked rhamnose (d 101.9), 3-linked rhamnose (d 103.0) and non-reducing ends of uronic acids (d 105.0). These results suggest that the polysaccharide produced by Gayralia sp. differs from those produced by Monostroma and Ulva species.