Water-soluble and exchangeable components of Vietnam acid sulfate soils were analyzed. Large amounts of water-soluble Fe, Al, Ca and Mg sulfates and sulfuric acid were found. Their amount and the ratio of the amounts of water-soluble ions to those of the exchangeable ions increased with the decreasing pH. The strong acidity at low pH (<2.7) was due to water-soluble Fe and Al sulfates. At pH 12.8. water-soluble Fe^<3+> disappeared and the contribution to the acidity of exchangeable Al^<3+> and Al sulfate increased. The amount of water-soluble Ca^<2+> + Mg^<2+> (17-44 me/100g) was higher than that of acid sulfate soils from other countries. The treatment with H_2O_2 caused the remarkable increase of water-soluble Fe^<3+>, Aｌ^<3+>. H^+ and SO_4^<2-> especially in subsoil layers. This indicates the high potential acidity derived from sulfides and the liberation of Al from organic matter.
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