Biological air treatment methods are an alternative to conventional treatment methods such as activated carbon adsorption and chemical scrubbing. An external loop airlift bioreactor has been utilized to treat phenol-contaminated air using Pseudomonas putida. Saturated air was found to be cleansed of phenol below the detectable limit because of the high mass transfer rate of the pollutant from the air and the high growth rate of Pseudomonas putida. The bioreactor was found to degrade over 99% of the inlet phenol at rates from 21.5 to 194 mg h(-1) at concentrations between 650 and 850 mg m(-3) of air. A model of the system is developed based on an initial transient period followed by a pseudo-steady state period. The simulations compared well with the experimental data.