The fate of ammoniacal-nitrogen (NH4-N) was studied in a lab-scale downflow reed bed system treating an artificial landfill leachate. Individual reed beds were saturated by the leachate, then drained and rinsed by tap water. NH4-N was removed by a two-stage process, adsorption onto the reed bed media followed by nitrification into nitrite-nitrogen (NO2-N) and nitrate-nitrogen (NO3-N). Decrease of the NH4-N level of the leachate was observed when the reed beds were saturated. By rinsing the beds, part of the NH4-N adsorbed inside the bed matrices was released into the rinse water. The presence of NO2- and NO3-N in the rinse water demonstrated that nitrification took place while NH4-N was retained inside the bed matrices. For artificial leachates with NH4-N levels of 150 +/- 5 mg/l, an average removal rate of 44% in a 3 h treatment was achieved. Mass balance analysis indicated that adsorption, transformations into NO2- and NO3-N accounted for 64, 4 and 24% of the NH4-N removal, respectively. This study also demonstrated that in general a greater rate of effluent recirculation around downflow reed beds gives higher NH4-N removal. (C) 2003 Elsevier Ltd. All rights reserved.