A dynamic model to describe ammonia removal in a gas-phase biofilter was developed. The mathematical model is based on discretized mass balances and detailed nitrification kinetics that include inhibitory effects caused by free ammonia (FA) and free nitrous acid (FNA). The model has been able to predict experimental results for dynamic operation under different loading rates (from 3.2 to 17.2 g NH3 h-1 m-3). In particular the model was capable of predicting the outlet ammonia gas concentrations as well as reproducing satisfactorily the gaseous ammonia concentration profile with time under FA inhibition and under non-inhibitory conditions. A sensitivity analysis showed that pH strongly influences the results of the model. © 2005 Elsevier B.V. All rights reserved.