- We tested the possibility of utilizing acetohydroxyacid synthase I (AHAS I) from Escherichia coli in a continuous flow reactor for production of R‐phenylacetyl carbinol (R‐PAC). We constructed a fusion of the large, catalytic subunit of AHAS I with a cellulose binding domain (CBD). This allowed purification of the enzyme and its immobilization on cellulose in a single step. After immobilization, AHAS I is fully active and can be used as a catalyst in an R‐PAC production unit, operating either in batch or continuous mode. We propose a simplified mechanistic model that can predict the product output of the AHAS I‐catalyzed reaction. This model should be useful for optimization and scaling up of a R‐PAC production unit, as demonstrated by a column flow reactor. © 2005 Wiley Periodicals, Inc.