A time and space dependent model was developed to analyse the performance of two-phase upflow and downflow fixed bed bioreactors and was applied to biological phenol degradation. The hydrodynamic parameters were determined from residence time distribution measurements, using an imperfect pulse method for time-domain analysis of non-ideal pulse tracer response. A transient diffusion model of the tracer in the porous particle coupled with the PDE model was used to interpret the obtained RTD curves. Gas-liquid mass transfer parameters were determined by a stationary method based on the least square fit of the calculated concentration profiles in gas phase to the experimental values. The analysis of the performance of fixed bed bioreactors used for phenol biodegradation shows that for the same operating conditions two-phase upflow gives much higher conversions. Gas-liquid mass transfer is the limiting step in the rate of phenol biodegradation when the inlet phenol concentration is high.