Lipase can be used for the synthetic reaction if the reaction is carried out in low-water media. The synthetic yield can be enhanced by continuously removing the water produced during the reaction. Many researchers have proposed several methods for water removal, such as headspace evacuation (1,2), pervaporation (3), use of molecular sieve (4), salt hydrate pairs (5,6), saturated salt solution, adsorption (4), and sparging of dry inert gas through the reaction medium (7). However, the continuous water removal might result in the enzyme inactivation when the water content is too low. Therefore, a correct water content must be maintained when biocatalysts are used in synthetic reaction. Especially, synthetic activity of the lipase is greatly affected by the water content or water activity (aw) (8,9). Generally, it is advantageous to use the water activity (aw) instead of water content or water concentration to characterize the water effects in the reaction mixture (10). In this section, we described the application of pervaporation and salt hydrate pairs to the aw control for the lipase-catalyzed esterification.