Several reactor designs have been described in the recent literature for continuous organic phase enzymatic esterification reactions. While these designs have excellent performance characteristics, there are operational constraints in their use. The present article describes a new reactor design, the gas-phase hollow fiber reactor (GPHFR), which does not suffer from any such limitations. The reactor consists of, commonly available, hollow fiber dialyzer modules with enzyme immobilized on the lumen of the hollow fiber membranes by ultrafiltration. Substrate mixtures are passed through the fiber lumens and subjected to esterification with a constant humidity gas phase recirculated through the shell of the reactor, acting as the medium used to control water activity. The simplicity of the device renders it suitable for use over a wide range of water activities, and its modular nature facilitates easy scale-up. The use of the reactor for the fixed water activity esterification of an equimolar mixture of dodecanol and decanoic acid has been described. Under optimum conditions the reactor was found to give yields of ester as high as 97%. In continuous operation the immobilized enzyme was found to have a half-life of about 70 days.