Hexyl-α-chymotrypsin, a hydrophobic derivative of the enzyme, is explored for the proteinase-catalyzed ester synthesis reaction with N- acetyl- l- tyrosine and ethanol. To guarantee the preservation of the enzyme activity and to allow for the extraction of the product in the organic phase, a biphasic system was used. The V m increased for the modified enzyme. This phenomenon was linked to the modification of the protein as demonstrated by its chemical denaturation with sodium dodecyl sulfate.