Two polysaccharides, GAPS-1 and SAPS-1, were isolated from the gel and the skin of Aloe barbadensis Miller irrigated with sea water for 3.5 years through a combination of anion-exchange column chromatography and repeated gel chromatography and their chemical characterization and antioxidant activities in vitro were investigated. GAPS-1 and SAPS-1 were composed of Man:Glc:Gal in a ratio of 120:23 and 296:36:1 with their molecular weight 1.74 x 10(5) and 3.97 x 10(4) Da, respectively. IR and C-13 NMR study of GAPS-1 and SAPS-1 demonstrated that the main skeletons of GAPS-1 was beta-(1 -> 4)-D linkaged mannose with acetylation at C-6 and C-3 of manopyranosyl and SAPS-1 was beta-(1 -> 4)-D linkaged galactoglucomannan with acetylation at C-6 of pyranosyl. Ina concentration-dependent manner, GAPS-1 and SAPS-1 were demonstrated to have strong scavenging activities against superoxide radical, moderate ferrous chelating effect, moderate scavenging activities of hydroxyl radical, moderate reductive power and moderate inhibition of lipid peroxidation. Furthermore, GAPS-1 exhibited moderate scavenging activity of hydrogen peroxide, while SAPS-1 exhibited weak scavenging activity of hydrogen peroxide. GAPS-1 demonstrated more effective antioxidant activities than SAPS-1. The more acetyl group in GAPS-1 molecules probably contributes to the activities. (C) 2007 Published by Elsevier Ltd.