The operation of a fixed bed biofilm reactor for nitrification colonized by Nitrosomonas europaea and Nitrobacter winogradskyi has been characterized in terms of steady state operation but also studying the dynamics of the system in front of disturbances of ammonium load. The nitrifying reactor is used in a biological life support system (BLSS) in which it is necessary to assure high conversion to nitrate and to avoid nitrite production. In order to enhance the knowledge on the system and also as a simulation and control tool, a mathematical model has been developed, calibrated and validated for both steady state and dynamic conditions of continuous mode operation. The results presented demonstrate the applicability of the model to the experimental system, providing a simple and useful mathematical tool to be further applied to the control of the reactor and to maintain the long-term stability of a loop of interconnected bioreactors in the BLSS. (c) 2004 Elsevier Ltd. All rights reserved.