We investigated the yield, gelation and sugar composition of water-soluble polysaccharides that had been extracted from soybean cotyledons under acidic conditions. SSPS were easily extracted at several pHs. The highest yield was about twice that reported by Morita et al., after extraction in boiling water, or by Kawamura et al., after extraction in hot water (90°C) with the stepwise addition of ammonium oxalate and 0.5% NaOH. Our results show that the average molecular weight was relatively high for both SSPS extracted in the weakly acidic pH range of 4-6 at 100-120°C and in the strongly acidic pH range of 2-3 at 40-80°C. The water-soluble polysaccharide solutions which were extracted under the former conditions remained fluid, but those which were extracted under the latter conditions gelled. However, the molecular weight distribution and sugar composition were similar for SSPS isolated by using both sets of conditions. Furthermore, water-soluble soybean polysaccharides were found to consist of rhamnogalacturonans, which could endure hydrolysis, and arabinogalactans, which were easily hydrolyzed under acidic conditions.