The kinetics of the oil/water two-phase reaction system was analyzed, and the reaction was carried out with the desulfurization of alkylated dibenzothiophenes (Cx-DBTs) using the desulfurizing microorganism Mycobacterium sp. G3. In the water-phase reaction system, the desulfurization activities were constant with respect to species of Cx-DBTs as substrates. However, the desulfurization activities in the oil/water two-phase reaction system against DBT, 4,6-dimethyl DBT, 4,6-diethyl DBT, 4,6-dipropyl DBT, and 4,6-dibutyl DBT were 49.0, 45.9, 11.5, 1.35, and 0.00 mumol g DCW-1 h(-1), respectively. The kinetic parameters for the degradation of DBT, 4,6-dimethyl DBT, and 4,6-diethyl DBT were also obtained (V-max values 90.0, 68.7, and 22.7 mumol g DCW-1 h(-1) and K-m values 0.21, 0.70, and 3.03 mM, respectively). The reason for the decrease in activity against Cx-DBTs of high molecular weight was a decrease in the V-max value and an increase in the K-m value, the latter being a particularly serious problem. Furthermore, the hydrophobicity of the substrate was evaluated as the capacity factor measured by high-performance liquid chromatography (HPLC). The correlation between substrate hydrophobicity and desulfurization activity indicated that the desulfurization reaction in the oil/water two-phase reaction system is greatly influenced by the hydrophobicity of the substrates. In addition, the influence of the solvent on desulfurization activity was examined, and it was found that not only the hydrophobicity of substrates, but also that of solvents, affected the desulfurization reaction. (C) 2003 Wiley Periodicals, Inc.