Packed bed hollow fiber membrane reactors were used to carry out organic phase biocatalysis at constant water activity. The performance of the device was tested by carrying out the esterification of dodecanol and decanoic acid in hexane. Lipase from Candida rugosa, immobilized on microporous polypropylene and packed in the shell space of the reactor, was used to catalyze the reaction. In situ water activity control was accomplished by pumping appropriate saturated salt solutions through the microporous hollow fiber polypropylene membranes. Water generated by reaction in the organic phase, pumped continuously through the shell of the reactor, was transferred into the bulk of the aqueous phase under the water activity gradient. The reactor performance was found to be strongly dependent on the controlling water activity. By carefully selecting this control activity it was found possible to obtain complete esterification. The water activity of the organic phase could be maintained very close to that of the saturated salt solution used. The reactor could be operated in the continuous mode for 100 h without any degradation in its performance.