A method for the removal of water and the control of water activity, a(w), during enzymatic esterification is the use of salt hydrate pairs. When this technique is used on a laboratory scale, the recovery and reuse of the salt are not critical. Potential problems, such as the reactivity of some salts, can also be overcome simply by substituting another salt. However, if this technique is to be used on a larger scale, economic constraints would require salt recovery and restrict the range of salts that could be used. In this article a twin-core packed-bed reactor - used for the esterification of an equimolar mixture of decanoic acid and dodecanol catalysed by lipase from Candida rugosa - which facilitates salt recovery and permits a(w) control without direct contact between immobilized enzyme and salt, has been described. a(w) control was maintained by using suitable salt hydrate mixtures in the inner core of the reactor. The substrate mixture was esterified by pumping it through the outer core of the reactor, which contained enzyme immobilized on a macroporous polypropylene support. Complete conversion, albeit at different rates, was obtained with a(w) buffering at 0.48 and 0.8 by using hydrates of Na4P2O7 and Na2HPO4.