A reaction-diffusion biofilm model was implemented to simulate the nitrification/denitrification performance of a lab-scale integrated fixed-film activated sludge (IFAS) reactor. The model was capable of representing the system performance, i.e. changes in organic load and decrease in sludge age. Furthermore, nitrification batch tests with sludge and carrier material could also be simulated successfully with the model. Model simulation revealed that the diffusive fluxes into biofilm depended strongly on substrate loading as well as sludge age. The microbial composition in the biofilm matrix was mainly influenced by the diffusive flux of chemical oxygen demand (COD) into biofilm. When COD removal started to switch to biofilm, heterotrophic bacteria quickly replaced the previously dominating autotrophic bacteria. Running a set of simulations with a range of effective biofilm surface area and different mass transfer coefficients revealed the strong influence of these two parameters on the IFAS performance. The analysis showed that both parameters were dominating factors for ammonium removal. The optimum mass transfer coefficient was in the range of 3-4 m d(-1) and the effective biofilm surface was around 63-88% of the theoretical carrier surface.