alpha-Chymotrypsin was used to synthesize a kyotorphin derivative in water-miscible organic media. Increase in solvent mole fraction deactivated the enzyme exponentially. Maximum synthetic activity was with a water activity in the medium of higher than 0.8 for all assayed solvents. The increase in solvent hydrophobicity (log Pi) (being Aw higher than 0.8) enhanced 2-times both the selectivity of the synthetic reaction, and the fluorescence intensity and emission wavelength of the enzyme, without dependence on the nature of solvent.