To elucidate the adsorption characteristics of lipases and to study the influence of the reaction conditions on the catalytic properties of lipases, the hydrolysis of decylchloroacetate by Pseudomonas fluorescens lipase in an emulsion reactor was studied as a model system. During the reaction the droplet size distribution of the emulsion was measured on-line using a particle sizer based on light scattering. Desorption experiments revealed that, at low surface coverage, the initial rate of reaction was not influenced by either the stirring speed or the organic volume fraction. Dilution of the reaction mixture during hydrolysis did not result in a decrease in activity. Based on these results, it is assumed that under the specified conditions adsorption of Pseudomonas fluorescens lipase is quantitative and probably irreversible. Based on activity measurements and assuming that only a monolayer of lipase is active, it is calculated that at saturation the emulsion interface is covered with 3 mg lipase per m(2). From these data the average interfacial area covered by one lipase molecule at saturation was calculated to be 1700-2100 Angstrom(2) per molecule. The emulsion was shown to be dynamic, e.g., during hydrolysis a significant increase in interfacial area was observed as a result of a shift in droplet size distribution to smaller diameters. Experiments indicated that both the formation of decanol and the emulgating effect of the lipase account for these observations. The formation of decanol also resulted in a dramatic decrease in hydrolytic activity. Taking interfacial tension measurements into account, it is shown that decanol accumulates at the liquid-liquid interface. As a result of this high local decanol concentration at the interface, a reversed reaction, alcoholysis, occurred.