The use of biofilters for the control of air contaminants in Advanced Life Support (ALS) systems is currently being investigated by the Waste Processing and Resource Recovery research team of the New Jersey - NSCORT (NASA Specialized Center of Research and Training). Ammonia (NH3) was selected as a test air contaminant as it presents special challenges to the sustained operation of a biofilter. Ammonia loading to the ALS atmosphere will likely be from waste treatment (biological treatment of human, plant and food wastes) and food processing operations. This NH3 has the potential of causing adverse effects on plant growth and humans.While studying NH3 removal and transformation in biofilters the following significant observations were made: (1) NH3 is removed through the action of nitrifying bacteria which oxidize it to nitrite (NO2-) and nitrate (NO3-); (2) NH3 is removed due to biologically mediated abiotic reactions when it serves as a base which titrates the acid produced during nitrification; (3) a significant amount of the inlet NH3 is converted to Organic-N, presumably through assimilation by microorganisms; (4) NO2 - accumulation in a biofilter can lead to the production of NO and N2O gases, but only a relatively small amount of nitrogen gets converted to these nitrogen oxides. The NH3 loadings of the 14 L biofilters tested were 250 mg NH3-N/day, with > 99 % removal sustained for over 100 days.