The yield of organic-phase enzymatic esterification reactions can be improved by continuous removal of product water. When water is the only volatile component of the reaction system, this can be accomplished by carrying out the reaction under a partial vacuum. The performance in such reaction systems can be further improved by employing a controlled leak of air into the headspace of the reactor. This improvement is achieved at a lower vacuum than would be required in an ideal evacuated reactor delivering the same performance. The theory of air-leak effects has been analyzed in this paper. Experiments done to verify this theory have also been presented. Air-bleed evacuated-headspace reactors (ABEHRs) can produce extremely high synthetic yields. For instance, during the lipozyme-catalyzed esterification of a solvent-free solketal-decanoic acid mixture, a yield of 96% ester was obtained in a reactor operated under a vacuum of 0.7 bar and a temperature of 50 degrees C when air at 20 degrees C with a relative humidity of 54% was leaked into the headspace.