Inhibition of nitrification by sulfide was assessed using sludge obtained from a steady-state nitrifying reactor. Independent batch activity assays were performed with ammonium and nitrite as substrate, in order to discriminate the effect of sulfide on ammonium and nitrite oxidation. In the absence of sulfide, substrate affinity constants (K-S,K-NH4=2.41 +/- 0.11 mg N/L; K-s,K-NO2=0.74 +/- 0.03 mg N/L) and maximum specific rates (q(max,NH4)=0.086 +/- 0.008 mg N/mg microbial protein h; q(max,NO2)=0.124 +/- 0.001 mg N/mg microbial protein h) were determined. Inhibition of ammonium oxidation was no-competitive (inhibition constant (K-i,K-NH4) of 2.54 +/- 0.12 mg HS--S/L) while inhibition of nitrite oxidation was mixed (competitive inhibition constant (K-i,(NO2)') of 0.22 +/- 0.03 mg HS--S/L and no-competitive inhibition constant (K-i,K-NO2) of 1.03 +/- 0.06 mg HS--S/L). Sulfide has greater inhibitory effect on nitrite oxidation than ammonium oxidation, and its presence in nitrification systems should be avoided to prevent accumulation of nitrite. By simulating the effect of sulfide addition in a continuous nitrifying reactor under steady-state operation, it was shown that the maximum sulfide