Protein engineering and kinetic prediction by linear free energy relationships are tools that can be used to rationally optimize enzyme function in organic solvents. Subtilisin BPN’ from Bacillus amyloliquefaciens has been used in a variety of organic solvents. The effect of the solvent and water content on catalytic efficiency has been determined on a number of substrates and kinetc relationships have ben developed. Site-directed mutants of subtilisin have also been studied and improvements in catalytic efficiency in nearly anhydrous polar organic solvents of at least one-order of magnitude have resulted.