Hydrothermal treatment of glycerin was carried out at 300 degrees C by using eight alkaline catalysts, including hydroxides of alkali metals, alkaline-earth metals, and aluminum. All alkaline catalysts promoted the formation of lactic acid or lactate salts from glycerin, except for Al(OH)(3). The alkali-metal hydroxides were more effective than alkaline-earth-metal hydroxides on the catalysis of hydrothermal reactions. Oil the hydrothermal conversion of glycerin into lactic acid, the catalytic effectiveness followed the sequence of KOH > NaOH > LiOH for alkali-metal hydroxides, and Ba(OH)(2) > Sr(OH)(2) > Ca(OH)(2) > Mg(OH)(2) for alkaline-earth hydroxides. Ail excellent lactic acid yield of 90% was attained on hydrothemal conversion of glycerin at 300 degrees C with KOH or NaOH as a catalyst. KOH was superior to NaOH as a catalyst since it worked at a lower concentration or within a shorter reaction time to obtain the same lactic acid yield. The hydrothermal conversion of glycerin depended not only on the hydroxide ion concentration but also on the metal ions of catalysts.