In this work, a model that considers the change in biofilm density at different biofilm depths was applied. Two correlations, describing the variations of the oxygen and substrate effective diffusivities as functions of the biofilm density and depth, were introduced. The model was validated with experimental data of oxygen concentrations measured with a microelectrode at different biofilm depths in a laboratory-scale rotating biological contactor (RBC) system at different rotational speeds (1.25, 2.75, and 5.25 rpm.), using municipal wastewater. The oxygen and substrate concentration profiles depend on the position on the biodisk. Comparisons between the COD and oxygen concentration profiles obtained with the model and those obtained considering the microorganism density and diffusivities to be constant were performed. The effects of the biokinetic parameters mu(max), K-S, and K-O2 on the COD and oxygen concentration profiles inside the biofilm were tested.