A 41 external loop inversed fluidized bed airlift bioreactor (EIFBAB) was constructed, characterized and tested for treating high strength phenolic wastewater. Expanded polystyrene beads (S.G. 0.713, 1.0-1.18 mm diameter) were used as supporting materials, for immobilizing Pseudomonas putida ATCC11172, in the downcomer of the EIFBAB. A unique feature of this bioreactor is the installation of a globe valve between the riser and the downcomer sections. By adjusting the valve opening, gas holdup in the riser and liquid circulation velocity were decoupled from the gas velocity. The effect of the extent of valve opening, under cell-free condition, on gas holdup and liquid circulation velocity was investigated for gas velocities, which ranged from 0.01 to 0.12 ms(-1). It was found that gas holdup in the riser could be increased by a minimum of 55% (at 0.1 ms(-1) gas velocity) to as much as 85% (at 0.01 ms(-1)) by closing the valve. The data for gas holdup were modelled using a power law correlation while a Langmuir-Hinshelwood kinetics model was used for the liquid circulation velocity. The latter was rationalized based on an energy balance approach. The advantage of the EIFBAB was demonstrated for batch biodegradation of phenol at concentrations of up to 3000 mg l(-1). It was found that at high phenol concentrations, biodegradation was oxygen limited, and the EIFBAB was ideal for increasing the gas holdup by closing the valve at higher gas flow rates while maintaining the same fluidization bed height.