Quantitative evaluation of the influence of biofilm density and diffusivity of substrates on stationary properties of a three-phase fluidized-bed bioreactor was performed. An aerobic process of biodegradation of toxic carbon substrate was analysed. The results of computation were compared with those obtained with the assumption of integral mean of the parameters describing the biofilm. The conditions were indicated at which the assumptions concerning constant values of the kinetic parameters in the biofilm are valid. The computations were performed for biodegradation of phenol occurring according to dual-substrate kinetics.