Simultaneous removal of NH 3 and H 2 S was investigated using two types of biofilters-one packed with wood chips and the other with granular activated carbon (GAC). Experimental tests and measurements included analyses of removal efficiency (RE), metabolic products, and results of long-term operation (around 240 days). The REs for NH 3 and H 2 S were 92 and 99.9%, respectively, before deactivation. After deactivation, the RE for NH 3 and H 2 S were decreased to 30 -50% and 75%, respectively. The activity of nitrifying bacteria was inhibited by high concentrations of H 2 S (over 200 ppm) but recovered gradually after H 2 S addition was ceased. However, the Thiobacillus thioparus as sulfur oxidizing bacteria did not show inhibition at the NH 3 concentration under 150-ppm conditions. The deactivation of the biofilter was caused by metabolic products [elemental sulfur and (NH 4 ) 2 SO 4 ] accumulating on the packing materials during the extended operation. The removal capacities for NH 3 and H 2 S were 6.0 -8.0 and 45-75 mg N, S/L/hr, respectively.