A water activity control system for enzymatic synthesis in organic media, for litre-scale reactors has been constructed. Water activity, a(w), is a key factor when using enzymes in non-conventional media and the optimum value varies for different enzymes. The control system consists of a water activity sensor in the headspace of a jacketed glass reactor (equipped with narrow steel tubes to introduce air), gas-washing bottles containing blue silica gel (a(w) = 0) and water (a(w) = 1), a PC to monitor water activity and a programmable logic controller (PLC) to control the water activity. The system was evaluted by adjusting water activity in the medium, with a defviation from the set point less than +/- 0.05. Synthesis of cetyl palmitate, under controlled water activity and catalysed by two different lipase preparations, namely, Novozym (R) 435 (immobilised Candida anarctica lipase B) and immobilised Candida rugosa lipase, were also performed. Novozym (R) 435 catalyses reactions very well at extremely low water activity while C. rugosa lipase shows low activity for a(w) < 0.05.