Alginic acid is a liner hetero-polysaccharide consists of D-mannuronic acid (M) and L-guluronic acid (G). The alginate, whose molecular weight and the mannuronic acid/guluronic acid (M/G) ratio are adjusted, are used extensively in food, pharmaceutical industries, and so on. In this study, we developed the rapid decomposition of sodium alginate (Alg-Na) using subcritical and supercritical water (abbreviated as subH(2)O and scH(2)O, respectively) microreaction systems, and investigated the controllability of the molecular weight and the M/G ratio by varying temperature and pressure. In scH(2)O, glucosidic linkages and hexose rings of Alg-Na were completely cleaved by hydrolysis to form unknown complex mixture. Whereas in the subH(2)O (523 K, 25 MPa), G-M and M-G linkages were hydrolyzed selectively. As the result, we successfully obtained the almost-pure guluronic acid homo-polymer and the mannuronic acid-rich water-soluble hetero-polymer selectively.