The advantages from a 4-1 external-loop inversed fluidized bed airlift bioreactor (EIFBAB) reported by Loh and Liu [2001. Chemical Engineering Science 56, 6171-6176] was synergized with preferential adsorption by granular activated carbon (GAC) for the enhanced cometabolic biotransformation of 4-chlorophenol (4-cp) in the presence of phenol as a growth substrate. This was achieved by incorporating a GAC fluidized bed in the lower part of the riser with the gas sparger relocated above this fluidized bed to avoid the presence of a 3-phase flow in the fluidized bed consequently providing larger gas holdup. Expanded polystyrene beads (EPS) were used as the supporting matrix for immobilizing Pseudomonas putida ATCC 49451, in the downcomer of the bioreactor. The hydrodynamics of the bioreactor system was characterized by studying the effect of the extent of valve opening, under cell-free condition, on gas holdup and liquid circulation velocity at different gas velocities and solids loading (EPS and GAC). The experimental data for gas holdup were modeled using power law correlations, while a Langmuir-Hinshelwood kinetics model was used for culation velocity. The bioreactor was tested for batch cometabolic biotransformation of 4-cp in the presence of phenol at various concentration ratios of phenol and 4-cp (ranging from 600mg l(-1) phenol: 200mg l(-1) 4-cp to 1600mg l(-1) phenol: 200mg l(-1) 4-cp) at 9% EPS loading and 2.8% (10g) GAC loading. The 4-cp and phenol biotransformations were achieved successfully in the bioreactor system, which ascertained the feasibility of the bioreactor. Biotransformation of high 4-cp and phenol concentrations, which was oxygen limited, was also effectively achieved by increasing the gas holdup in the riser. This was possible in the current EFBAB system because of the synergistic effect of the GAC fluidized bed, the globe valve and cell immobilization by EPS. (c) 2005 Elsevier Ltd. All rights reserved.