A mathematical model was developed to describe the biodegradation of phenolic wastewater in a fixed biofilm process. The model was solved using a combination of orthogonal collocation method and Gear's method. A laboratory-scale column reactor was conducted to verify the model system. The batch kinetic tests were independently conducted to determine biokinetic parameters used in the model simulation while initial thickness of biofilm was assumed so that the model simulated the phenol effluent concentration curve very well. The removal efficiency for phenol was 94%. At a steady-state condition, the phenol concentration obtained from experimental data was slightly higher than that obtained from the model simulation. The approaches presented in this paper could be employed for the design of a fixed biofilm reactor system for the biodegradation of phenolic wastewater in petrochemical and oil refining industries.