Lipase-catalyzed (Lipozyme IM 20 and Novozyme SP 435) esterification reactions in 5-methyl-2-hexanone were studied in order to investigate the practical application of a water activity control system consisting of a semipermeable silicone tubing and saturated salt solutions. The system used successfully in different reactor configurations [batch, continuous stirred-tank reactor (CSTR), and tubular] was described. The dimensions of the tubing could be calculated given the diffusion coefficient of water (D-aw) through the tubing, the amount of enzyme, and the maximum accepted increase in water activity. Tubular reactors with continuous water activity control were constructed. The enzyme preparation was packed in a silicone tubing which was submerged into a saturated salt solution. A reactor with Lipozyme was successfully used for two months without any decrease in product yield (72% conversion at a(w) = 0.33). By connecting a second tubular reactor (containing immobilized lipase and molecular sieves), the yield increased to 95%. A two-stage tubular reactor was used to catalyze esterification in pure substrates (equimolar amounts). The yield (after the second reactor) reached equilibrium (95% ester) at a water activity of 0.33.