Enzymatic transesterification of guanosine having low solubility against organic solvent was examined. For the transesterification between guanosine and divinyl adipate catalyzed by alkaline protease from Bacillus (Bioprase), DMSO was added to DMF to increase the solublility of the nucleoside, and the conversion rate of guanosine to the vinyl guanosine ester was less than 30%. To overcome the reversible inactivation of enzyme by hydrophilic organic solvents, the reaction was carried out with 10% (v/v) water. The transesteritication reaction was effectively catalyzed in DMF/DMSO in the presence of water and the conversion rate increased ca, 70% after 7 d reaction. The result shows that the water effect of Bioprase would be a useful method for the synthesis of low solublility nucleoside esters.