6-O-Lauroyl mannose was synthesized at 50 degreesC through condensation of mannose and lauric acid using the immobilized Upase Novozym (R) 435 in four water-miscible organic solvents, acetonitrile, acetone, 2-methyl-2-propanol and 2-methyl-2-butanol, with various water contents, The equilibrium conversion significantly depended both on the kind of solvent and on the initial water content of the solvent. The apparent reaction equilibrium constant for the condensation Kc, which was defined based on the concentrations of substrates and products, also depended on the kind of solvent and could be correlated with the relative dielectric constant of the solvent.