Benzaldehyde lyase (BAL; E.C. 22.214.171.124) from Pseudomonas fluorescens Biovar I catalyzes the reversible formation of benzoins from aromatic aldehydes, and, moreover, the coupling of aromatic with aliphatic aldehydes yielding derivatives of (R)-2-hydroxy-1-phenyl- propan-1-one (R)-HPPs), which are important chiral building blocks. In this paper, we report on the development of a reactor system that allows the selective production of substituted (R)-HPP-derivatives. The reaction systems yielding (R)-1-(3-chloro-phenyl)-2-hydroxy- propan-1-one, (R)-2-hydroxy-3-methoxy-1-(4-methoxy-phenyl)-propan-1-one, and (R)-2-hydroxy-3,3-dimethoxy-1-phenyl-propan-1-one were investigated. A kinetic model optimized by batch experiments was developed, for the description of both batch and continuously operated reactors. This model was used to describe the HPP production in a continuously operated enzyme membrane reactor. The reactor type used combines the advantages of high conversion and excellent selectivity with high space-time yields and total turnover numbers of up to ttn = 43,000. Products were obtained in high yield on a gram scale. Biotechnol. Bioeng. 2007;96:835–843. © 2006 Wiley Periodicals, Inc.