To enhance the Chromobacterium viscosum lipase (glycerol-ester hydrolase; EC 3.1.1.3) activity for the reaction of water-insoluble substrates, the AOT/isooctane reverse micellar interface was modified by co-adsorption of a non-ionic surfactant. Polyoxyethylene sorbitan trioleate (Tween 85) was used as the non-ionic surfactant and olive oil as a water-insoluble substrate. An appreciable increase of lipase activity was observed and at higher W-o values (where W-o=molar ratio of water to total surfactants of the micellar system) there was no sharp fall of the enzyme activity such as a typical bell-shaped profile. The kinetic model for the lipase-catalysed hydrolysis of olive oil in AOT/isooctane reverse micellar system was applied to the enzymatic reaction in this mixed reverse micellar system. It was found that the predictions of the model agree well with the experimental kinetic results and that the adsorption equilibrium constant of olive oil molecules between the micellar phase and the bulk phase of the organic solvent is smaller in AOT/Tween 85 mixed reverse micellar systems than in simple AOT reverse micellar systems.