In enzyme catalysis there is a great interest in finding suitable organic media for less water soluble substrates in order to increase the substrate concentration and therefore the reaction rates. These requirements are fulfilled by using w/o-microemulsions as reaction media. The influences of pH, temperature, water concentration and the kinetic parameters of Candida Rugosa Lipase in a nonionic w/o-microemulsion with a surfactant of technical grade, Marlipal O13-60, are presented. In an example the enantiospecific esterification of racemic menthol with propionic anhydride using this nonionic microemulsion likely to be affordable in large scale applications is shown. For a continuous process an ultrafiltration unit is attached to a reactor within a loop. In this way, the reverse micelles containing the enzymes can be separated from the oil, containing the product, and reused afterwards. (C) 2001 Elsevier Science Inc. All rights reserved.