The benefits of controlling water activity, a(w), during enzymatically catalysed synthesis reactions? such as reverse-hydrolytic reactions promoted by lipases, are now well recognized. Numerous techniques for controlling a(w) in the laboratory and their implementation in continuous reactors have been discussed in the published literature. However, in enzymatic interesterification reactions, such as acidolysis and transesterification, it is not appropriate merely to maintain the a(w) of the reaction system at one value since the two stages of the reaction, namely the cleavage of the original acyl bond and the formation of a new one, are best carried out at different levels of water activity - the former at a high a(w) and the latter at a lower one. The use of a continuous packed-bed hollow-fibre reactor has been described in this article for carrying out solvent-free acidolysis of ethyl laurate with octanoic acid with in situ a(w) control, using air that has been pre-equilibrated with saturated salt solutions to the desired a(w). At a single optimum(a(w) = 0.54), the highest steady-state conversion to ethyl octanoate was 32%. However, it is possible to obtain a steady-state conversion of 46% by operating the reactor with a step change in the water activity, from an initial value of unity to 0.23.