Sodium alginates were extracted from brown seaweeds (Cystoseira schiffneri) collected from a Tunisian island (Kerkennah, Sfax) in different seasons (December, April, July and September). The structural features and antioxidant properties of C. schiffneri sodium alginates (CSSA) were characterized. Micro-elementary analysis showed the absence of nitrogen and sulfur in alginate fractions. All alginate isolates were high in uronic acids (47.4-66.4%) and ash (24.3-39.4%). ATR-FTIR, NMR and circular dichroism data showed that all CSSA were of the polyguluronate-type. Weight-average molecular weight determined using high performance size exclusion chromatography showed variations ranging from 4.49 to 1230 kDa. Thermogravimetric analysis suggested that CSSA were stable until 400 • C except for that collected in September, which was observed to only be stable until 200 • C. Antioxidant activity of CSSA from different seasons was measured using DPPH• radical-scavenging, reducing power and Fe 2+ chelating ability assays. Antioxidant potential of CSSA varied significantly with respect to the season and the main factor controlling antioxidant properties was the molecular mass.