The ratios of mannuronic acid (M) to guluronic acid (G) of sodium alginatesextracted from 9 species of brown seaweeds, Sargassums and Laminariea, collectedalong the coast of China, were determined. Among them, Sargassum siliquastrum andS. henslowianum have higher content of G and lower M/G ratios of about 1, and La-minaria has higher content of M and higher M/G ratio of 2. The M/G ratios are lowered from 1.13 in the original alginate to 0.16 in thefractionation product after oxalic acid hydrolysis for S. siliquastrum, from 0.82 to0.14 for S. henslowianum and from 2.01 to 0.09 for Laminaria. But those of thefractionation products by MgCl_2-EtOH precipitation are less lowered. The average molecular weight and degree of polymerization of the sodiumalginates tested are generally decreased after oxalic acid hydrolysis to about 2,000 and10, respectively. The inhibition efficiencies of sodium alginates from differient species of brownseaweeds prior and after oxalic acid hydrolysis on the absorption of ⁸⁵Sr in the gastro-intestinal duct of rat indicate that the sodium alginate from Sargassum siliquastrumgives the best efficiency, enhancing from 66% prior hydrolysis to 78% after hydro-lysis, followed by that from S. henslowianum, incroasing from 56% to 64% and by that from Laminaria, from 27% to 56%, respectively. The inhibition efficiency of sodium alginate from S. siliquastrum on the absor-ption of a mixture of ¹⁸³Ba, ⁸⁵Sr and ⁴⁷Ca indicates that it is remarkably effective on¹⁸³Ba, increasing from 14% before hydrolysis to 77% after oxalic acid hyrolysis.