The effect of ultrasound irradiation on activity and particle size of a subtilisin-catalyzed interesterification reaction in an organic solvent was studied. Across a range of levels of water addition and irradiation amplitude, sonication failed to increase reaction rate compared with stirred systems. Sonication was shown to be effective in reducing particle size, but the effect of any reduction in diffusional limitations on reaction rare was exceeded by an inhibitory effect of the treatment. This inhibition was found to be irreversible upon cessation of sonication and aqueous assay of activity, or recycling of the enzyme into a stirred reaction system.