The use of air treatment biofilters for the control of trace air contaminants in advanced life support (ALS) systems is currently being investigated by the Waste Processing and Resource Recovery team of the New Jersey - NSCORT (NASA Specialized Center of Research and Training). Ammonia (NH3) was selected as a model compound because it presents special challenges to the sustained operation of a biofilter; additionally, ammonia' is a contaminant of concern to ALS. The special challenges that NH3 removal presents to biofilter operation are due to (dynamic changes in the biodegradation environment which result from the accumulation of ammonia and its metabolic (biotransformation) products. This accumulation degrades the quality of the environment eventually limiting and eliminating the desired biological activity.This paper contains inform&ion on the development of a mathematical model for a nitrifying biofilter. The model presented takes into account the physical (advective and diffusive transport), chemical (acid-base chemistry and vapor-liquid equilibrium), and biochemical (biodegradation stoichiometry and kinetics) nature of the system. This model is being developed to provide a better understanding of the system dynamics than can be achieved through experimentation alone and to take the first step toward the development of a biofilter simulation model which may be used in system studies and for biofilter design and control.The work completed to date includes calibration and development of the physical and chemical aspects of the model. The calibration results indicate that the experimental biofilter acts as a plug flow reactor with a small amount of mechanical dispersion (D = 0.325cm2/s) and that the rate limiting step for mass transfer of ammonia into the biofilter media appears to be diffusion through the aqueous phase.