Pseudomonas fluorescens strain BTP9 is used as biocatalyst to produce vanillic acid from vanillin. Several two-phase reactors were investigated and compared to the corresponding one-phase systems in order to optimize this bioconversion. A water-dodecanol system was set up. High cell density and entrapment of the cells in alginate beads are two characteristics of this reactor. With this kind of reactor, vanillic acid productivity was increased (3.4 g/L/d) compared to the one-phase reactor, with a conversion rate near 80%. Vanillic acid yield and productivity strongly depended on pH, stirring rate, cell immobilization, and substrate concentration.-d?